Does he regularly have anxiety issues?
The reason I ask is because Hannah does. As you probably remember, she has bloated/torsioned, and had two bouts with hemorrhagic gastro-enteritis. She has had chronic stomach issues for over a year and a half now.
Finally, in April or May, when we noticed her anxiety levels getting worse, we consulted with our vet who put her on an anti-anxiety medication.
It's done wonders for her stomach. She's also on a prescription diet and other medications, but the anti-anxiety meds are what I've noticed to help the most.
I know when I'm stressed, my stomach is the first thing to be affected.
Anyhow, good luck with his recovery. I know how frustrating it can be, and how sad it is to see them miserable.