I used to feel the same way. I don't know if this will work for "him" here, but it works for me.
Let it go.
WHatever it is, whatever it was, whatever it might be. Let it go. Try hard, let the chips fall where they may. Then let it go.
Many times, one feels pissed because he takes it that way. Don't take it that way anymore. Let it go.
Because the alternative, to be angry like that, is not good. It's exhausting to be so angry like that. it really is. So let it go.
I'm not kidding around and I'm not mocking anyone. This really does work for me.
Jake