Well I hope it isn't anything too serious, he definitely looks like a sweety. :) I wasn't assuming he was badly bred either, just I know I had that experience but hey, it happens to the best of them sometimes. :(
I went to see the little guy today and he is looking better and has more sparkle in his eye. I guess the last wormer was what he needed. She will worm him again in 4 weeks as per instructions.
As for his fetlock -- it also looks better, but now the other hind is doing the same thing, though not as badly.. HOWEVER, I looked at his feet today and they are not properly trimmed. Somehow my friend and I both didn't looking at that possibility before -- she: because they were just done by a farrier about 2 or 3 weeks ago and me: because I'm too opinionated on the matter and try to keep my mouth shut unless asked. However, when she recognized that he was now bringing both back feet too far underneath himself, the feet became an issue.
So, what this all means, I'm not sure yet, but once his feet are done we'll see. That should be fun because the farrier beat on him last time and today when I picked out his front foot, he fought and reared. My friend and I won though!!! And he then stood quietly. We're going to take our time doing his feet with lots of praise so he gets over it.
He looks to have a wormy belly to me which would explain why he is looking better after his deworming. You can try feeding him cinnamon (a teaspoon to start) as its a natural dewormer. You can feed that to him everyday with his feed while keeping on schedule with his regular deworming. Don't use the same dewormer either everytime, use something different. He does look thin and his feet don't look too bad although that fetlock could be caused by unbalanced feet. Hard to tell from the pics. He's a cute little guy :) hope everything works out for him! How old is he?