Hi, I'd like to help you with your question.
It definitely sounds like your son has PTSD. It is a natural reaction to going through a life threatening event such as your son did.
If therapy did not help, he may need to try another therapist. Sometimes, finding a therapist is like finding a family doctor. You need to see a few before you click with one. He can ask his doctor for a referral or if he attends church, his pastor can help. He can also look on line at http://www.findatherapist.com/.
In order to stop the nightmares, he needs to treat the original problem, which is the trauma he suffered. Cognative Behavioral therapy (CBT) can help him address his thoughts about the trauma. Here are also some very good links to help him find more treatment options:
Your son needs to work out his feelings about his trauma in therapy and relearn how to feel calm. He also can learn relaxation techniques that will help him relax anytime he feels anxious, and especially before he goes to sleep.
It takes time, but PTSD can be overcome with some good therapeutic work and practice. Medications can also help. His therapist can let him know if medication may be needed in his case.
Please let me know if you have any more questions I can help with,