I too get the same thing when I want to masterbate myself. Just before or even the thought I sneeze. My g.f thinks that maybe I am allergic to her or something. I just tell her it is not her.
Some people release histamines during orgasm, which is the same chemical released during an allergic reaction. Some sneeze, some get itchy and some get red blotches on their bodies. It's not that rare and can be considered normal for him.