P.S. He says he's trying to do better on his grades yet he fails to turn stuff in or he tunes out when he doesn't understand. The schools are not very helpful when it comes to kids who are not motivated to learn.
You mentioned that professionals have suggested that your son has attention deficit disorder. That may be an accurate diagnosis (except that Ritalin® wasn't helpful), but I think the bowel problem may be playing a role. If any of us were constipated all the time, we'd probably focus on the discomfort and have a hard time paying attention to other things. His low level of motivation may be a sign that he's a bit depressed (who wouldn't be?).
It sounds to me like this problem has gone on for a very long time, without any real resolution. I think there's a need for more intensive intervention. Have you considered having your child checked into a hospital or clinic that specializes in bowel and bladder problems? They could observe him over a week or so, taking a closer look at his diet (liquid and solid intake), his emotional status, family involvement, and the effects of counseling. He could continue his schoolwork from the clinic; you could even request that he be evaluated by the school problems clinic staff while he's there. Your son needs a multi-disciplinary evaluation by professionals who can compare notes and come up with a successful treatment plan. I know this sounds a bit drastic, but it seems a real shame for this to continue, when a solution may be available.