We use Strategy and beet pulp, senior feed for one of our older horses. You also might want to consider a daily wormer for the persistent/drug resistant worms. The barn owner did that with the little mare I have and she plumped out overnight. Also might want to think about one of those SmartPak gut supplements. I'm getting ready to get one of those for our horses because I recently learned that somewhere between 60-90% of horses in hard training can get ulcers. I think that can cause weight loss, too. Also, don't know if this is important but might want to check to see if his teeth need floated. I think that might make it more difficult to put on weight. I've used corn oil and olive oil before. One of the barn owners says Flax seed oil is the way to go. There are also Omega supplements - I think you want to get the Omega 3's mostly. Also in QH's and Appaloosas need to watch out for this mitochondrial disease that can predispose them to tying up. That can affect what you feed them.