I agree with everyone to ditch the grain ASAP. Grain does not settle well on an ulcerated stomach.
1. Can you feed the feed pan stuff three times a day? Horses with ulcers really benefit from that.
2. I would keep the horse on ulcer meds all winter until the horse has green grass to eat.
3. Rice bran, even though 22% fat, is healthy fat and provides the horse with cool energy. Or if the horse is an easy keeper, feed it straight timothy pellets.
4. Top dress the rice bran or timothy pellets with a liquid vitamin and NOT Red Cell. It only takes one ounce of liquid for the horse to get everything it needs. I always add in some warm water to help get more liquid into the digestive system. It's important to get as much liquid in them as you can without forcing the issue
I use Tuttles Liquid 747. http://www.y-tex.com/pdfs/747.pdf
A gallon will last two months and is only around $22 if you can find it locally. Or pay the shipping fee and buy it from Valley Vet or someone similar.
That being said, my 26-1/2 yo Arab gets gas from the Tuttles. He has gastric ulcers so I feed him "Daily Start by EquiShine" It is a pre-probiotic. He eats Triple Crown Sr and is doing well.
You're going to have to experiment but the main thing is to get the horse with ulcers off all that grain and also plan on keeping him on a pre-probiotic for the rest of his life. Just like people with ulcers can't eat certain things, neither can horses