I think he needs work Bit hard when they are only two eh?
Given that he is still quite young and was allowed to become drastically thin at an early age he lost not only fat but also some muscle mass, you can really notice this around his topline. Now that he is gaining weight, he is doing it from the bottom up, so right along the top of his neck and back still looks a little 'drawn' which makes his belly look bigger than it really is by contrast. This effect can happen with the young 'uns anyway as they go through awkward growth spurts. Also, notice the change in his coat condition? Always a good indicator of good health. Not the be all and end all of course but certainly a good sign.
As you say, he has had vet attention and been thoroughly wormed, I would just make sure he keeps looking as healthy as he does and enjoy watching him recover into a beautiful boy!