PTSD can end up either in avoidance or an obsession, as others have mentioned.

And quite frankly - you've said twice that you think he was like this before he went overseas. How on Earth would you know - you've said you have the impression of it, which says that you didn't know him before. Don't judge. You don't know if he was like that before or not, and you wouldn't know unless you asked his family... which would be very inappropriate as you aren't friends.

If, as you said, he won't go and recieve help, the most you can do is to be supportive and let others know to knock off the judging. It doesn't mean that they need to be subjected to stuff they're not comfortable with - you can all just assert that you aren't engaging in X conversation and then leave.