To me it sounds like you're going about it the right way. For all their strengths as a breed the mental side can be an issue. They like a set routine, the same area to patrol, the same people and animals to see - anything else sets off the guard instinct. Your boy is obviously adjusting to his new surroundings. From my experience they're not an overly aggressive dog, he wont suddenly attack you but if you back him into a corner and he feels like he's no other option he will bite. So he needs time, just keep going with the socialising and it will be worth it.
As for food, whilst they appear a large dog take away the fur and they're surprisingly slim. As such you don't need to go overboard. I would think 2 cups of dried food combined with meat/veg would be fine. My boy would get the dinner left overs from my 3 kids with some dried food.