Sorry see this is old. I have had the same issue with my OTTB. I say high chance of ulcers, if he has yet to put on any weight, mix corn oil in with beet pulp, feed sweet feed, make sure there is lots of free choice hay. If weight does not begin to return, contact your local vet and request a bottle of Omaprozole, this will help rid the ulcer. It is around $160 a bottle, that can last about a month and a half. It will be the best buy you ever make, becuase it will save your horse and you alot of stress. Make sure though as he is putting on weight he also puts on muscle to prevent hightened chances of lameness. Best bet, 15 minutes of straight trot on the bit and lots of turning, as well as 10 minutes of walk-trot-halt-trot-to what ever you want next transitions to gt him using his back.
Has he been seen by a vet lately, and has the weight loss been gradual or sudden? I would have his teeth looked at, firstly, and proceed from there.
What does he get as forage? If he's had laminitis in the past (assuming it was grass founder), then he's probably not on 24/7 turnout. If that's the case, you should consider increasing his hay ration.
One thing I would recommend is adding a non-molasses beet pulp (soaked) to his supplements. Low in sugar, and will add water to his diet. Since BP is easily digested, it's good for putting on weight, and on top of that it helps to set up a really good environment for the natural flora in the gut. Some horses will not eat it straight, but in my experience most will.
You may look into putting him on a low-starch, low sugar feed. These feeds will have a low NSC (non-structured carbohydrates). Good for horses prone to laminitis. You can find plenty of information online by searching for NSC in horse feed.
Sorry I can't be of more help. Again, I suggest speaking with your vet and having his teeth and mouth looked at, and also having his ears, nose, and under his tail examined carefully. Also, it would be in your best interest to have him scoped for ulcers.