What do you think of my TB's conformation?

well, he's not over at the knee, which is good. he has a good sized backbone. but he looks a little thin, like you said...i think he needs to just get worked. the picture of his backside looks like he lost a lot of muscle. so backing up exercises would help gain muscle in his back end. his neck is perfect size, not too long or too short. i think, overall, he just needs to get into shape, gain some weight & muscle, then he'll look marvelous. also, in one of the pictures he looks slightly pigeon-toed. maybe ask your shoer about that? it's not a huge deal, unless he has an extreme case of it, which i don't think he does.