I would not use molasses as a regular feed. No horse needs that much extra sugar, even if they are not prone to laminitis. It can cause other digestive and behavioral problems. You wouldn't give your child twinkies for lunch just because he's calm and not overweight.
You could soak your feed, maybe? Or try to find feed he likes better? A lot of horses really enjoy alfalfa, and it's easy to find it in pelleted form. One of my horses hates beet pulp, but the other really really likes it soaked in hot water. There's got to be something common that he would like. Also, most ponies don't need the calories of extra feed, so maybe all you need is a way to get him to eat the supplements. Most have a pelleted version, and I have found those are much easier to feed. They are pretty easy to disguise in some alfalfa pellets. If he needs more calories and he's a picky eater, there are some herbs like fenugreek that are supposed to smell really appetizing to horses that you could put in some feed. A higher quality or higher quantity of hay would probably put weight on just as well, though.
Tug on anything at all and you'll find it connected to everything else in the universe.~John Muir