Maybe he was trained a different way than what you are used to, or maybe he's just a bit confused in a new place. Maybe you can call the previous owners and talk to them about the dog.
If the dog is not well trained, and is five, I would NOT start with him as if he was a puppy. Puppies are immature mentally as well as physically, and cannot focus or work for more than thirty seconds. They are not physically strong and one needs to be very careful with them physically. They get tired quickly especially larger breeds that have a lot of growing to do.
Some dogs have been trained SOME, but don't know your commands or how to do all the things you may want. These are really not that hard.
If the dog has never had any traiining, he may not 'know how to be trained' and you may at first need to keep your training sessions very short. But no dog really learns well with a one hour training session once a week, or the way my friend did, with a three hour training session once a month!
The more often and more briefly you train any dog the better they do. Three minutes 10 times a day is better than 30 minutes once a day. Done the right way, training is no burden at all and the dog learns much faster. Don't train hard, train smart. Train seven days a week. Anyone can spare a few minutes out of their day. Keep the leash, treats close at hand and keep the dog nearby and it is no big deal at all to take a few minutes.
If the dog is being rehomed, it is possible that he was rehomed because he has a problem - some Goldens are very distractable and nervous - not all breeders are carefully selecting the parents of their litters.
If that's the case don't get entirely fooled into not doing anything for months, by a very submissive or fearful response from the dog. It's not always the right thing to baby a dog. Sometimes a cheerful, 'let's do this, girl!' works out a whole lot better. You have to feel your way along on this issue, and even a hint of pressure freaks some people out (the dog's behavior is usually far more flexible than the owner's!). Sometimes the more you baby a dog the longer the problem goes on. For example, a long time, chronically very skittish dog we got to train, was a new woman after two days of being told, 'so...do it anyway'. The first day she was lying on the bed with our lot with her paws in the air, having a nice snooze (was supposed to be terrified of other dogs) after a 'gang walk' with three other dogs. She was sitting on command after one good yank on the leash. You can go overboard with 'tough love' so be VERY careful with this part of it. Some people don't have much success with this sort of thing - they get overly punitive or in too much of a hurry. There's a very, very fine line between skillfully challenging a dog versus scaring the bejaysus out of it.
If a dog is afraid of something, treats help. For example, we got one who was afraid of loud noises. In one day, he was wagging his tail every time we slammed a door or yelled. He wound up going to a fireworks display and wagging his tail through the entire thing! No, I'm not kidding. Every time a firework went up he jumped up and demanded a treat, LOL. He was the hit of the party.
You can teach a dog just about anything. I taught one of mine to bark every time someone hooked a fish at the pier. We called her 'Radar', LOL.
Some goldens are so busy being friendly or being scared or noticing every dog or person that goes by that they are hard to get their attention. It's also possible that the previous owner simply was one of those, 'let him be free, Man' type owners who simply didn't train the dog, and that the distractable behavior is simply an untrained dog.
You'd think it would be easy to 'retrain' a dog but if it has never been taught to do anything at all has missed that message they need to get when they're young 'people are fun and it's fun to listen to them and do what they ask', it's almost as hard as dealing with a dog with an inbred bad temperament. It can require major work to get an older dog to 'make up for lost time'.
If you're asking here what to do, you need help, and I don't think that any website can offer the kind of immediate, 'ok, he reaacted that way, so now you need to...' or 'no, don't jerk so hard, jerk lighter on the leash, see his reaction?' type thing. Find a dog training class and go to it with your dog. Get immediate and in person help.
General guidelines?
Get the dog a comfortably sized (LARGE) crate, and keep him in the crate when you are not training him. Take him out of the crate and work with him with a chain training collar and treats. Put the collar on (right side up) and put a leash on it. Hold the long end of the leash looped in your hand. Go to a quiet, undistracting area without other dogs, people or outdoors distractions. Say 'sit!' and immediately jerk up on the collar and press down on his rump until he sits. Say 'sit' once and try to get a reaction immediately. Don't say anything else other than 'sit'. Don't chatter.
Some dogs get it better if you hold a treat above their head, they shift their weight back onto their hind quarters and sit easily. Others jump up to grab the treat. Do whichever works best with your dog. Get a response before you give a reward.
Rinse, repeat.
Teach the dog to come. Play with him on a leash. Throw him a ball a short distance, say, just get him a little away from you. Say 'Buffy, COME!' in a cheerful, loud voice and haul him in with the leash. When he gets near you, give him a treat and say 'good boy!' Don't say anything else.
You may first need to teach him his name. Get him on a leash. Say his name, 'Buffy!' and when he looks around, give him a treat. You can teach him first by saying his name and giving him a treat. When he hears his name, he should look around toward you. Don't teach him to come to just his name. His name really means 'listen! something good is coming up!' The next command might not be 'come' at all, it might be 'stay!' because a car is coming and if he 'comes' he'll be run over!
Never punish the dog for coming to you. Always praise him if he comes no matter what just happened or what he did before that. Or he won't come to you.
When he chews on something he should not chew on, say 'NO!' (Don't say his name, the name is never connected with punishment). If you have to, squeeze his mouth so his lips press on his teeth, so he gives the item up.
If he is not housebroken, take him outside and praise him when he goes. Take him to the same place each time. Some dogs learn better if they are given a treat after they eliminate.
All punishments and corrections have to be geared to the dog's temperament. Very sensitive dogs need light corrections, and dogs that are less sensitive need more forceful corrections.
Sometimes gun dogs like retrievers have the 'I can't hear you' problem. They don't respond until the owner gets really worked up and is yelling and smacking, and then when they do finally respond they are frightened and cowed, they cringe and wet the floor.
It's very important with these dogs, to realize that they DO need to be corrected, but they need a different kind of handling. They need to learn to key into the owner more, all the time. Their problem is more that they are distracted than that they are really bad dogs. Teach the dog to pay attention to you and focus on you. Treats work well. so does this little trick. Most of these dogs 'follow' a treat really well. Get the treat in front of their face and move it around up to the side of your face. Hold it there and say 'watch me!' and eventually, when you say, 'watch me' the dog will look at you, and you can give him a command.
Dog training means repetition and being very, very consistent and using the exact same commands and exact same body language for the exact same thing. It means teaching things in steps, not in big leaps. It means having control of the dog with a leash when you give a command. ALWAYS remember - do NOT give a command unless you can enforce it. Don't say, 'Buffy COME!' when Buffy is totally untrained and loose in the yard. Only use commands when you can enforce them (the biggest mistake most people make - it's like running after a car in neutral when the parking brake is off yelling, 'stop!')
Don't 'converse' with a dog when training it, especially the older untrained dog who has learned for years that the human voice means nothing of interest to him. Use single words and always use the same words.
Always follow a word with a 'logical correction'. For example, if he doesn't respond to ONE command to come, he immediately feels the leash pulling him toward you.
Make him think that living with you is a 'game'. The key of the 'game' is to beat you to that leash correction. 'If I respond to that command right away, I outfox her! Not only that I get a treat for outfoxing her! Heh heh!'
I met a lady who every time her dog didn't respond to a command, she said, 'WRONG!'
Don't, please. It's 'wrong', LOL.
Also, don't use treats every time. Once the dog gets the idea, try going through a few steps, then a few more, before giving a treat. 'Intermittent reinforcement' is the most successful type of reinforcement of a behavior.
The dog has no concept that not responding to a command is 'wrong' or that not sitting and not coming are all 'wrong'. They don't generalize or think like that. That just teaches him, 'oh, no, the person is saying something', when he should be thinking, 'oh boy, the person is saying something, something good is coming! I better really tune in! There's something in this for me!'