Looking For A Probiotic For My 23 Year Old Son.

Looking for a probiotic for my 23 year old son. Since thru hiking the Appalachian Trail (6 months) his stomach has been out of whack. Since the hike he has also mountain biked the Continental Divide Route (3 .5months) and the Trans America Route (90 days) all these trips in about a 2 1/2 year time frame. His diet was not great during those times and now is suffering with bouts of diarrhea and stomach discomfort. He is now willing to figure out what is going on! We're looking for a natural daily supplement any thoughts or suggestions?
Thanks!