Experiencing Pain In Stomach, Dr. Says Constipation. Had 4 Clearl Liquid Like Bowels. Causes ?

My 16 year old son, has been expericing pain in his stomach for a couple years now, and we have seen the GI doc for a year now, and all she says is constipation , but he goes regurally, he is on no medications , but he seems to always have pain in his stomach....well this morning he woke and was little constipated then in the last hour he has had 4 bowel movements, and he says the are clear liquid that looks like water....you have any idea what could be causing this?