Welcome to the forum!
I also have PTSD from bullying in middle school- it's not uncommon. I really like the fact that you have taken the step to come to this site, it shows that you are accepting of his diagnosis.
First thing you can do is be there for him; if he wants to talk about his trauma, let him talk about it, and if he keeps talking about it, let him continue, it's part of the healing process. If he doesn't want to talk about it, don't push him to. Only a therapist should ever push someone to talk about trauma, there's a risk of re-traumatization if others do so.
Second: Recognize his triggers. You can recognize his triggers once you recognize his reactions. Does he start shaking? Do his eyes get wide and then he seems to just stare straight ahead? If he does this he's having a flashback. Once that happens, you need to figure out what may have caused it. Certain places, people, or words may be triggers. Try to limit his exposure, to them. If you know that something is going to happen before that is a trigger, prepare him for it. If he's prepared, that can help to lessen a flashback. Part of the reason they can be so bad is because the trigger is unexpected, if we're prepared, we know how much of the exposed trigger we can handle, and so we can regulate that anxiety around it for ourselves.
Third: This may sound really strange, but if he asks where he is, who certain people are, if he texts or emails you weird things or mean things, or calls you and says mean things or weird things, if he's speaking to you and says weird things or mean things, and then when you bring it up later he says he doesn't remember, he's not joking and he's experiencing/ experienced dissociation. It's something that happens where the person gets confused because of triggers- If he can't "fight" (flashback) or "flight" (run out of the room) he will "freeze" (dissociate maybe) Just tell him very calmly "You're 22 years old. You're here, we were just (doing whatever the two of you were doing). You're with me. I care about you. Nobody is doing anything mean to you right now." It's important to remain calm and reorient him back into the present. You may have to repeat it two or three times, but it's necessary. Sometimes dissociation is caused by flashbacks, or it may be follow by flashbacks.
Hope this helps! And again, I'm very happy that he has the support that he needs. Sufferers need supporters who are not in denial themselves, as the sufferer may be.