Welcome curlingmom,
Are you keeping a food dairy? We started with low residue diet. Chicken, white rice, eggs for example and slowly added things in one at a time add a new food for a few days and see how he reacts if all is good add another. If he has problems you go back and maybe try that food again later and see if you get symptoms again. You will soon know what foods affect him. Certainly some are forever off the menu for us and a lot of other on here - popcorn, seeds, nuts.
What medications is he currently on? When my son is feeling well we don't really have issues with food and he can eat pretty much whatever, although we try and stick with clean eating although when life gets hectic and sports, practice, etc get in the way we tend to stray.