As best as you can tell, he has neither gained nor lost weight in any significant amount, is that true? If so, that is good. Well, gaining weight would be better for a still young snake, but gosh, he hasn't been eating, so you can't really expect it. The important thing is he hasn't lost weight.

As Greg said, he is probably feeling insecure. Are the sides of his tank covered with a dark material? If not, cover at least 3 of the sides.

You say 4/5 of his tank is full. To you that might seem crowded, but to a young BP that might seem like a lot of empty space. Try putting balls of loosely crumpled newspaper anywhere in the tank that you can fit them.

Also, you've gone through quite a lot of things in a relatively short period of time. You might have more luck with establishing a feeding routine, and sticking with it regardless of whether or not he eats. It would be best to stick with what he was being fed and eating successfully before, but if that isn't an option, then live mice are probably your next best bet.