If he has been cleared medically then the cause could be psychological. No one can be sure of that unless he receives an evaluation. He could however benefit from the support and guidance of a mental health professional. First you have to introduce the idea of talking to a therapist or projecting the possible benefits is really pointless. I have one suggestion that could be valuable to consider. You (and him) can access therapy online and he would not have to face a therapist with such a sensitive issue. If you Google etherapy and ecounseling you can actually access real therapy that is done by webcam or even IM if he doesn't want to face a therapist. Everything involves actual therapy where you get a time and you can talk without them even seeing you. This might be your best hope.
If this has been helpful press accept