He looks pretty good to me, but I agree he needs muscle on his neck and hindquarters. What does he eat? How often do you work with him?
My OTTB has problems keeping weight on too, he has the same good day/bad day issues. We had a bad drought where I live this summer, so the grass was basically non existent and he looked like a rack of bones with skin draped over. I started paying extra for good quality hay and I switched his feed over to a low sugar/high fat grain. Also, I upped his exercise routine to help him gain more muscle. I started riding him through the hilly fields next door, slow and steady at first, but we worked up to a canter over the summer. He looks tons better now.
"A rider who would trade partnership for obedience
will have to settle for neither."