What a pretty boy! I am partial to palomino's (and have a little soft spot for them)
Looks to me like he has a short neck, well-defined withers, a steep shoulder, but good, strong legs. He is wide in the chest and has a nice deep girth. He is of good weight, but, as you said, could use some more muscling- especially on his hind quarters. He looks to be happy and healthy buuut
I would like to question his feet. My old farrier used to leave my horse's feet looking like yours but I've recently acquired a new farrier who is currently working to repair the damage :S
I can see that Champ has flares, and the toe has been left long, causing his heel to wear down. I'm no farrier, and don't claim to know anything more than that, so I apologize if I am wrong but I just thought I'd share that bit of information!
From my own research, and what he's told me, having the weight on the sole of the foot like that (rather than the outside) can lead to bruising and abscesses, and having their heel worn down and the toe longer causes extra strain on the tendons (potentially leading to bows and such). It is also unbalanced and can make it difficult for your horse to perform.
I'm not going to pretend to be an expert on the matter at all though so it would be helpful if someone else (possibly another farrier on the forum) could post :)
Again, sorry if I'm wrong, but that's all I can see P: