Ooo, lactose intolerance. They make you self diagnose that. If he wants to save a trip to the doctor's, the thing you do is just go off dairy for a week and see if you feel better. The other thing they suggested when I was going through my own troubles was not enough fiber, so they had me take crazy amounts of Metamucil for a week. The dairy thing turned out to be it. I hear lactose intolerance is very common amount adult Asians, so yeah, statistically speaking, that might be the place to start investigations.