What a challenge you have before you!
I've read all of the responses. It sounds like you might want to explore the effect of all of the meds. It may be that he is getting treated for things that the meds are creating. (Meanwhile, there is a blood test for celiac disease. I recently had a biopsy for it and they then asked for a followup blood panel. A blood test is far less invasive than a biopsy.)
If you are able to get a full review of your son's issues, perhaps at the Children's Hospital in Boston as some suggested, that sounds like the best route. Maybe it would even be worth it to ween him off all of the meds and to start over. It's just a thought but worth exploring.
I have a 12-year-old who still has nighttime wetness and occasional daytime wetness. He perpetually plugs the toilet with his BMs. (We are currently addressing his constipation to see if that helps with the wetness.) He tends toward ADHD but is not enough to medicate.
Children are complex beings and even more complex if they have issues, whether medical or emotional. Is your son able to relax in life or are his issues a main concern for the entire family? Sometimes our concern for our children can become all-consuming and can contribute to the situation. Is that a possibility in your family? We realized it was happening in ours and we began family counseling a year or so ago. It helped all of us to learn some new ways of dealing with various medical and emotional challenges.
My best of luck to you! Sounds like you are the advocate your son needs. And you are right in seeking answers. Be persistent but also remember to relax. The answers will come with diligence and patience.