May I suggest you ask him to come, don't, DON'T ask about what happened, which I think you would not anyway. He wants I am guessing to remember the old days and get on with his life. I have a younger, much younger friend who I treasure he and his wife dearly. My friend was in the military in an elite unit. I NEVER ask him what he did. He does not want to talk of it. He has not said he does not, but I know from conversations with him and so forth THEY do not want to talk of it. Just sayin', kinda' like the guys coming back from any war, if you will.
There's absolutely no way I'D bring it up, but we were both pretty active in the JROTC while in school, and the majority of his HS friends were in there with us. Most went on to join up in one branch or the other. If I were in his shoes, I don't know if that'd keep my mind off it or not.