If he asks you to leave you alone, it's best that you respect that decision and leave him be. Individuals with PTSD will sometimes become overwhelmed or triggered by something and want to be left alone to either process things or feel safer - it can sometimes last days or weeks, even months. It's a common occurence.
If you feel truly concerned about your friend, wait a few days and see if he comes around. If he does, try and see if he'd be comfortable in talking about what you should do should he desire to isolate again, and what kind of things he'd find helpful in the future. But this all depends on how close you are to him. If you've been a friend - or perhaps an acquaintance - for a short ammount of time, do not be surprised if he has no desire to speak about his disorder with you. Some individuals just aren't comfortable in talking about their PTSD.