My foals always did great on the same thing I fed mommy, Purina Equine Senior OR Triple Crown Senior/Complete feed and beet pulp. (Make sure to ad oats with beet pulp if you choose to feed that so he gets the right Calcium:Phosphorus Ratio)
That and free choice hay would get him fattened up.
He looks nice! The only things that stood out to me were his neck appears a bit long and thin, his rear end/legs look (for lack of better words) "post-y" not bad - however critiquing a growing horse is always a toss up as to what they mature into with age and muscle :) Enjoy him!!!