Mouth flare ups post full mouth extraction for stomatitis

I adopted my cat with severe stomatitis and FIV+. The shelter did a full mouth extraction on him 3 months ago. He has been so much better than he was with teeth, however he still sometimes avoids his wet food and will meow in pain when eating. He still is showing inflammation in back of his mouth. Gabapentin helps for now but has anyone else experienced this? He saw my vet 6 weeks ago with the plan to continue to monitor to see if it would get better in case he was just taking longer to heal but no improvement. Any recommendations on ways to help outside of vet care such as specific supplements, diet, etc?