There are a few things to consider but without knowing what you are feeding it is hard to say exactly what might help the most. I'm a big fan of stabilized rice bran, there are a ton of them out there. I'm not a big fan of corn oils as another poster mentioned they weren't either and would use an omega3 oil if I wanted to go that route. It has the same benefits plus additional benefits that vegetable oils cannot provide. Filling them full of feed or oils isn't always the sure fire way solve a weight problem though, often there are digestive issues when you are doing everything else right.
A couple of questions first, what are you feeding? Does he show any signs of ulcers or hind gut acidosis? Does your vet offer any suggestions?
A good friend who is also our equine dentist was having a similar problem a month ago and was on a good feeding program and had tried many avenues with the help of his vet. We suggested he try EquiShure
and he saw results in a few weeks. Read the article and see if you think it can help you are call and chat with someone there if needed.