I had a friend whose child had this - even at age 14 the child was wearing adult diapers, having bathroom emergencies & accidents, and more. She was doing what you are doing and it just wasn't working. They did every doctor, every specialist, behavior modification, and on and on - such that the child's life revolved around intestinal issues and bathrooms! The intestines were so messed up that the child never got the "signal" to go to the bathroom, never knew when she was ready to go and never knew when she was really "done" either. No fun at all.
The challenge is to find something that will restore the body's natural balance and not be something that you "dose" him with every so often. He can't tell how impacted or clogged he is, and you can't either. Drugs are going to treat the symptoms and not the problem.
We have found fantastic results with a research-based food science company called Reliv International (www.reliv.com) - I'd love to share more info with you. You can put your child on a couple of delicious and balanced shakes a day with a fantastic fiber product (both soluble and insoluble fiber at the same time), and establish the balance that is supposed to be there. Within a few weeks, you should have relief for your child! No drugs, a U.S. gov't patent, and total safety, developed by a science team headed by the country's top expert in pediatric nutrition, Dr. Carl Hastings (he worked on the development of Enfamil and ProSoBee infant formulas and also was hired by Weight Watchers to oversee the nutritional make-up of their food line). Reliv gives you a lot of support - it's so opposite of buying a product off the shelves and trying to go down this road by yourself. I would love to help you help your son, and yourself!