Yes, it happens to everyone and no, it doesn’t matter. But that’s not what your poor partner wants to hear after a short-lived sex session. Don’t make a joke out of it either, just lighten the atmosphere with something like “that works out well, I’ve got a train to catch in the morning.”
You could also try smiling, throwing your arms around him and saying something reassuring and deep: “that was a lovely taster of what’s in store for us next time.” If you have the impression that it happens frequently, wait until you get closer and suggest a trip to the doctor. The results will be impressive and you’ll be the first to benefit!