severe stomach pain after hard workouts(running)

My 17yr. old son has started to experience severe stomach pain after hard workouts or meets for cross country.  This occurs if he drinks water during practice (even small amounts) or after running when he eats or drinks something.  He's been running for 6 years and has never had this problem before.  We have tried different things, not eating high fiber foods, taking out dairy, not taking IBprofen, nothing works.  His doctor gave him Prevacid to take, but that only helped for about 3 wks., now the pain is back.  He also has some diarhea (diarrhea) and gas.  Any ideas would be helpful.