Why is my 12 year old getting kidney stones?

My son is on his 3rd kidney stone. He has meds that are support to work but the doctors dont know why he is getting those. I was just wondering if anyone knew or wanted to guess. He is one a special diet and he drinks a lot of water. But sometimes he has pain any way. We usually give him meds and a hot pad on his back and he chugs a lot of water, and that usually works but sometimes not. He also has a lot of gas to burping and farting a lot and when he gets home from school he is so so tierd. Maybe it is the tests there doing and the stress. why is this? PLEASE! this is important