maybe he loves to watch the scene from superman 2 the one where the astronaut gets his space suit ripped and dies in space well when an astronaut rips his space suit he poops his pants. Maybe he wants to become an astronaut who rips his space suit and poops his pants and dies space while wearing whit sneakers new balance to be exact.
I'm guessing that since you're dealing with a teenager with this issue that he is also disabled? This doesn't sound like a deliberate act on your son's part. I would take him to see a pediatric gastroenterologist. There are some medications meant for IBS, which is what this sounds like to me. There are antispasmodic medications, one of which is also a sublingual medication. A gastroenterologist will be able to guide you through which meds to try.
I would also try taking him to see a naturopath or a nutritionist. I have IBS with constipation, so my IBS issues are different than what you're dealing with. But, I do know that my naturopath deals with a lot of people with digestive issues, especially things like IBS.
I would suggest finding a GI doc and a naturopath who will work together, practicing what is referred to as integrative medicine which is taking the best of conventional medicine and the best of natural medicine and merging the two together.