I would try just feeding him a fuller diet (add grain or forage feeds to his diet as well as free choice hay) without the supps first. He is growing and developing, which are both huge taxers of any diet. You won’t get a full tank’s worth of mileage on half a tank of gas. Can’t build muscle with no food. He will loose/under-develop muscle mass by trying to support other organs. Bodies priouritize the fuel that enters, what needs it gets it so there better be enough to go around the whole boat. ALso consider the drain of coping thru the day’s activities, being ridden regularly, heat, bugs, social issues good and bad, weather, critters that run thru the barn/field at night, all that stuff that we don’t really think about has a huge affect on how well your horse absorbs what he’s fed.
He might NOT be a hard-keeper, just growing and his body is using up what he’s getting and he’s not getting enough to cover all the bases, thus, he looks/is lean. For now, unless some odd health issue arises, just feed him a more nutrient-complete diet. The above statements are pretty accurate: hay alone for a growing horse is NOT enough. Not to push Triple Crown but their Growth formula is very similar to their Senior blend and can be pretty effective when added to hay.
Sure there's right and wrong but mostly there's just a whole lotta different.