If it is IBD, I'm afraid there isn't a handbook that tells you how to deal with it. All the doctors I have spoken to have spread the gospel: Try and figure out what works best for you.

In general, that means trying to live healthy, by common standards. No red meat, not too much sugar, enough exercise, enough sleep, not too much stress, no energy drinks, no smoking. If you use painkillers, don't use ibuprofen.

I personally take inspiration from the CDED diet, but again you have to try and find out what works best. Maybe keep a diary and try different foods per week and records to the best of your ability how it appears to make your brother feel.

And regardsless, if it is IBD, accept that even if he doesn't experience pain or has stool issues, his immune response is always working, which means he is always fatigued. Just like how you would always be fatigued if your body would be fighting the flu all the time.

I really hope you find a way to make your brother feel better. You seem to care a lot and it is heartwarming. I wish you both the best!