I think his weight is fine. If anything he looks too fat. I definitely wouldn't feed him more than his norm.
His coat looks a little dull but I would not worry AT ALL because he is clearly shedding and that can happen.
Here in MA the horses are just starting to shed, and only some. I wouldn't worry about that.
I do see a slight difference with the pointiness of his bones (technical, no? lol). I wonder if maybe he hurt himself a little sprinting around? When they're worked up like that, esp for a long period of time I'd be surprised if he didn't.
I would get a chiro out asap and then a vet.
If he dropped weight so drastically (maybe cause he hurt himself?) he may also of lost muscle mass and it changed the way he looks, with an older horse that might be difficult/impossible to get back.