He looks a little skinny, but not bad. I believe it's healthier for a horse to be a little skinnier than ideal, rather than a little pudgier, just like with people.
I vote for giving him a ration balancer
Purina is usually very easy to find and inexpensive. Their ration balancers are Enrich 32 (for horses on grass pasture/hay) and Enrich 12 (for horses on alfalfa)
Nutrena is a little better brand, also a little more expensive. Their ration balancer is Empower Balance.
I feed my horse Triple Crown 30% Supplement, but it can be hard to find. Even though I have several feed stores in my area, only one carries it, and I have to call ahead and make sure they have it in stock before I go get it (they usually have to have it transferred over from one of their other locations).