Ah, I think people are mistaking him being thin for just being narrow. He still has a bit of growing up and "unfolding" to do, especially through the front end. We had a yearling come in with less than great nutrition and she is similar to yours - narrow through the chest. But 13.2hh at 550lbs is not too far off. Lily was 550 when she was 13hh, now she's 14hh and 650 (she's being fitted though).
If you're concerned I would find a grain with a high protein/high fat ratio. Ours is a 12/10. Our yearling on light work is on 3/4 a scoop and my yearling being fitted is on 2lbs of that. But your guy looks good! A huge improvement since the last time we saw him, his hair coat looks really good. Maybe when you up his feed he would benefit from some gentle fitting? Some brisk hand walking should help build up his body.