Being that it has already been three weeks, you really need to have a doctor check him out and make sure his electrolytes are ok. Have him assessed medically if you are worried. Then.....if that checks out........
Start by talking to him mom. Sometimes, the bowels are very much emotioally related. Find out what is going on his life, any changes? Hormones? A Girl? Kids at scool? New to a new school or grade level? If you have a hard time talking to him, find someone who can. He may be very private about his owel issues. Sometimes, watery stool can even be a sign of constipation.
I am sure there is a good reason for this upset in his digestive system.
But be on it and make sure there is not a medical cause first.