There is no way to tell how his body will react. If his body was having a reaction to the foods because of unknown or hidden allergies/triggers (meaning once he stopped eating those foods, he felt better), then he will probably have a reaction within a period of time if he goes back to eating those foods, maybe right away, or eventually awhile when his body can't fight it anymore.
Personally speaking, I went on an anti-fungal diet and got well after 8 years of UC. Once I was well for about 6 months, I decided to throw caution to the wind, ate what I wanted, added alcoholic drinks to my diet, and eventually had a relapse. That took a few years to happen. Went on the diet again, and within days was better again. I know from that experience that I cannot eat like the way I used to before I got UC. I now eat those "trigger" foods with caution and in moderation. I find I don't even crave the old junk foods (soda, sweets) like I used to anyway. My tastes have changed. I can have some potato chips, sweets now and then, but I don't feel so well afterwards. Not UC symptoms, but just blah! My body is telling me it doesn't like it anymore. I do eat any vegetable and fruit now (no problem), but limit my grains (including corn), dairy, and still cannot tolerate yeasty foods.
If he tries to re-introduce foods slowly, and eats them in moderation, he will probably be able to tell if they are foods he will be able to enjoy once again. If he feels OK, then maybe once in a while would be fine. If not, then he probably won't be able to eat them again. Which is worse...not eating those foods, or suffering with Crohn's symptoms? Only he can make that decision. If it was me, I wouldn't push my luck, at least not too soon.
Is he frothing at the mouth to try these foods again? :)