That is WONDERFUL news.
I think that there is something about being given up on and not being taken care of properly, or given sufficient attention, that can really change a dog (or even a person). Our dog had a change like that too - he loved to run, but if given attention, a job, and enough exercise, he completely changed his attitude. Let him know very quickly where it was OK to put his energy, what he was supposed to do, and where he was NOT to put his energy. But, it takes the right person and the right time to be able to see that and take the chance. Good on you, Bee!
I also noticed that he was PERFECT for the first couple weeks, and then as a teenager, in a month or so he started to push the boundaries a little... That's when your reactions are key. Basically, from what I can tell, the dog has learned the rules, knows the drill, has now completely accepted his new place and home, and is just testing out the boundaries a bit. A sense of humour and a consistent approach is what's best, and Bee, we both know you have those, and your Ben is in GREAT hands!
I can't help but wonder if maybe part of his food aggressiveness was simply that he was hungry. Now that he got dewormed and some good food in him, he's not starving and fine! Our dog was like that. I could never get him enough food, and finally when I just fed him all out for 3-4 months he gained weight and is much more calm. I was already feeding him 2.5 times what the food said for his age and weight, but it too 6 for 3-4 months for his metabolism to catch up and then slow down.
By the way, how much does it cost to feed a GP per month?
Welcome home Ben!!!