A lot of people swear by pure Aloe Vera Juice (from a reputable health food store) but three gallons worth never did a thing to help my Arab. I ended up spending $150 for 30 days worth of Omeprazole
You can try Ranitidine and see if it works. WalMart's brand, "Extra Strength" - 150 mg per tablet.
My Arab is 13.3H and about 840 lbs. He gets 20 tablets daily, crushed up and mixed into his supplements. Supplements also include alfalfa pellets.
He doesn't need ulcer help when the grass is full of nutrients (the same full of nutrients that will founder my other two horses with metabolic issues
This time of year, generally until Spring grass, I will feed him some type of meds for ulcers.
I mention the Ranitidine because it's a non-prescription way to see if gastric ulcers are really the issue; if they are and they aren't severe, the Ranitidine should help.
My Arab is a long 26 so I have also recently put him on "Daily Start" by Equishine; a pre-probiotic. Even though there are pre-probiotics in his Triple Crown SR., it isn't enough. You might consider adding a quality pre-probiotic to help the digestive process
Hope this helps and good luck