Can you set up an outside place for him to stay, when you are not able to watch him?
He cannot pee in the house when he is not in the house!
And every time he pees outdoors, he gets a little more practice at doing the right thing.
You could try using a dog door. Give the dog only a SMALL space inside the house (enough to lie down comfortably, and no more.) Give him a bigger pen outside, that is appropriate for peeing. Of course you would need to teach him to go through the dog door. But this lets him go out whenever he needs to, no matter how often that is. If the only problem is how long he can hold it, this will solve the problem. If he also has a problem with where he thinks he is allowed to pee, this may still help but will not be a complete solution.
I have noticed that dogs like to pee in the same kind of place they are used to peeing (grass, or carpet, or dirt, or snow, or whatever.) And the ability to hold his pee for longer should increase as he gets older. So if you set up a way to manage the situation for now and keep him from peeing in the house (like an outdoor pen, or dog door, or taking him out every single hour all day long), then the problem may solve itself over the next few months. He will get more in the habit of peeing on outdoor surfaces, less in the habit of peeing on indoor surfaces, and will be able to hold it for longer between breaks. (I do know that "just a few months" can feel like FOREVER when you are living through it!)
The usual potty training advice says to keep the dog in a small space, because he will hold his pee to avoid soiling his "den." For this particular dog, it seems he is already used to peeing where he lives. So you might try always keeping him in big spaces (preferably outside), so he can move away from his pee, and get used to being clean all the time. If he is in the house, this will not help keep your house clean-- it will just change whether the dog lies in the puddle of pee or can move away from it.
I also agree with the suggestion to get him tested for a urinary infection. If he has one, you want to treat it before it gets worse, and it may make the potty training much easier. And if he does not have one, you will know it is not the problem.