I lost a lot of friends in Iraq, I was a civilian contractor, I got to know the young soldiers over there very well. I get very angry when anyone disparages any American, or person of another country that lost their life, in the worst terrorist attack in history, or in the world war on terror, and yes it is a world war. I was witness to what the news media called the worst loss of life, since the 911 attack, the suicide bombing of the dining facility in Mosul. The terrorist that blew himself up in the dining facility came VERY close to getting me. As it is he got himself and 43 other people, one of them was setting across the table from me, before he went to go get ice-cream. So I can feel personally how you would get angry that someone would disparage and say the offensive things that were said. Just remind him that, this is an international war, and that while America had the worst attack, and has lost the most lives in this world war, this fire of war is spreading fast and hotter in Europe, and that at one time, he might be on the subway, or the plane when the next mass attack happens. But when it does, he can feel safe that the U.S. will be there with both financial, and technological assistance. Then don't worry about it, you can not change people's minds, and if you let them get to you they win.
Then up a Chog and stomp the ever living dog snot out of him, don't let him know that he got under your skin.