This is more common than you think.
There are exercises to strengthen the pelvic floor - try asking a yoga or pilattes instructor for this information.
Also, have him take time in the evening to unwind. To read to relax, avoid too much tv/video games as they get him in his head and out of touch w/his body's signals.
My son has a couple of hours in his room everynight where he can do anything he wants as long as he is learning or studying something. He can read, study music, work on electronics, no tv/video games - real hands on learning. We started this when he was about your sons' age, at first he was a little resistant, but then he started getting into it and now he counts on that time to do his own learning. It has been a way for him to get to know how he learns and develop his own interest. It has helped him be aware of his energy and his body, which are good things for everyone. Boys tend to get in caught up in their heads and quite time where they can study how they want to is very interesting to watch. It is often a blend of drawing/doing/creating/learning and moving around that seems to help him know himself. It has been great for his confidence and he doesn't miss the tv at all. He does however enjoy sharing what he has created/learned.
Help him become more aware of his body and slow down the mind.
Also, there is a homeopathic that can be helpful for bedwetting.
Help him to recognize that he is much more than this situation.