B.C.
What has he been eating?
Lot's of things can be going on.
Food poisoning is a possibility but so is food allergy, chronic appendicitis, colitis, diverticulitis, irritable bowel, ulcer, reflux, gall bladder trouble, stomach/colon cancer, etc.
It might be trivial or it might be serious - he should see a gastroenterologist and find out what is going on.
If it is something serious, the earlier you catch it, the better.
If it's trivial, the peace of mind knowing it's not serious is worth it for both of you.
Make an appointment and drag him into the doctors office.