All the excellent food/supplements/hay in the world won’t make one bit of difference if the horse is not feeling secure, that is, is he in a pasture with anyone else? Where is he on the pecking order? Is he alone? All of these factors can make a huge difference.You say your horse is on 24/7 turnout. If he has to be hyper-vigilant (constantly on watch) he will only use up his calories trying to protect himself. Someone once asked me about a very similar situation to yours and when I went to visit the property I noticed that the horse was alone. There was no other horse to keep watch when the person’s horse wanted to rest. Think how herds function with a lead mare or stud always around to keep watch for predators. Domestic horses kept outside have not lost this need. I suggested they put him with a “friend” who would not push him around. 2 months later they contacted me to tell me that he had put on weight and was much less stressed.This is just one example of why behavioral reasons are often the reason horses fail to put on weight.