He can only be diagnosed by a professional. Self diagnosis is unreliable at best and dangerous at worst because even with a traumatic event that fits the Criterion A definition, there are still an array of disorders that could result, and many have similar symptoms. To boot, many non-trauma disorders share whole clusters of symptoms. And the treatment will be very different for some of these. That's why misdiagnosis can be so challenging and harmful- how you approach the problem varies a lot even if the problems look similar on the surface.
If he's not willing to work on his own healing, there is nothing you can do but try to educate him about the value of seeking help, that being a mental health consumer does not make him weak, and that he will have your support in whatever ways are appropriate for you to offer while keeping boundaries. But you can't do the work for him. Often people (with any disorder, mental or physical) have to hit rock bottom for that wake up call to get help and treat it seriously.
