I've had Crohn's for over 40 years, and as you have mentioned, it affects everyone differently. Generally, if he tolerates foods, he can eat them.

Most of the time I eat a something similar to the Mediterranean diet. But there have been times where all I have been able to tolerate is white bread and chicken. There have been times where I have had a hard time eating enough calories, and other times when that has not been a problem at all.

Talking to a dietician can help, but there isn't any "one size fits all" for CD.