Jeremy, first rule, and most important is never, never ask if he killed anyone. Period. If I could have a beer for every time I have had had that asked of me, I would have free beer for life. I once had an obnoxious person ask me that in front of a group of his friends, trying to be cool. My reply to him, as I looked directly into his eyes and said in a very, very low voice was "not enough. Always room for one more". He shut up pretty damn quick
If he wants to discuss those matters with you, he will on his own time and terms. Most I know will not, especially to someone who is more or less a stranger. Besides, unlike the misguided mentality many non-vets or those who are absolutely clueless have, not every vet has, and a good majority were never actually in the position to do so anyway. There is a considerable difference between "having served in a war/combat operation" vs "having fought".
Just ask him general questions that you wouldn't mind answering yourself. What did he do before the service, how long was he in, what are some of his fondest memories, favorite places he went or saw, funniest moment, etc.. Were there any regrets, were there things he would do differently if given the chance. What did he do afterwards, and so on.