carbonated => gas, which can give pain, especially if you're already a bit tender. Same goes for other foods that cause gas (cabbage, peas, beans)
caffeine => dehydrates, which is tough on your mucous membranes when they're already struggling.
And then there are mechanical irritant foods (e.g. nuts) and foods that bulk up make your intestines work harder, which again can give pain. So try going low fibre for a while to go easy on those inflamed intestines.

The good news is, that once the meds have got his guts in remission, he can try tasting his favorite foods again. I've been staying low fibre for months now, but lately I'm ok with the odd tangerine or handful of nuts or a latte at lunch or ... whatever. I just don't overdo it. Yet.