I would think you could add a little ( around a pound or so) of alfalfa cubes/pellets. I know Alfalfa gets a bad wrap, but it's actually pretty low in NSCs, just is high calorie and calcium imbalanced so shouldn't be a large part of the hay diet. BUT...if you use it to balance the bran (phos imbalance), it would help add more calories and variety to his diet. Alfalfa is also rich in many minerals and vitamins. It's not a bad hay, it just shouldn't be the ONLY hay, and if your horse is fat, the extra calories are bad, but I took a class and all the reserach I turned up just showed that the calcium issue and the high calorie thing (for already obses horses it could be the "straw that broke the camels' back" for laminitis, but unless suddenly added, is not really a high founder factor in moderation). It's mineral content is why it's often the carrier ingredient in supplements, as well.
I'm pretty sure I just made an unpopular suggestion, but I trim some IR horses and they do fine with a pound or two of alfafla. They also tend to add Flax to their diet, as the lack of green grass shorts them of that healthy fatty acid (one of the Omegas and I seem to get them backwards) .
Black Oil Sunflower Seeds, about a cup a day, also make a healthy variety, and add a sheen to the coat (more Omega fatty acids). The more variety you get in your horse, the happier he will be with that low sugar diet.
There is a supplement available through United Vet Equine called Carb-X, you should look into it. Just google them and check out all their supplements geared toward IR, Cushings and overweight horses to help prevent laminitis. THey contain large amounts of certain minerals they need with their conditions. There was a lady with a couple of fat ponies, and she gives them Carb-X as their "grain" each day, and their feet are looking excellent (they also get trimmed every 4-6 weeks, instead of the every 12 the last farrier recommened since they didn't break off (rolls eyes))