My 15year Old Grandson Has Had Severe Diarrhea For Over

My 15year old grandson has had severe diarrhea for over 2 months now. Dr has done lab work and tested for lactose intolerance and cannot find anything. He has broken 2 bones in the last month playing football. I know this is common in that sport but he has played for many years now and never broken any bones so I just WO dated of the two could be related in some condition that the Dr has not yet considered.