From my understanding I believe the symptoms are diahrea or constipation, gas, bloating, overall upset tummy. They can vary widely. What he's experiencing is not normal and I would suggest that parents check it out. Could be many things or it could be nothing, but better to know. Celiac could be a concern, too. I wouldn't turn a blind eye if it's an ongoing thing.
Funny, I say that and I've answered a question I've been asking myself for a few months now! I've been debating having my own daughter checked out for very low weight and on/off diahrea and on/off tummy aches. My mother and I both have celiac, so that's always on the back of my mind. Now I should take my own advice, I guess