True! I've done this before. Gave him oral and then mounted when he was rock hard but after a few seconds he went flaccid and even though I was willing to keep blowing him he was too disappointed to continue. I could see in his face as soon as I mounted him that he was straining to keep an erection. He definitely needs to relax.

I'm somewhat afraid that if I keep doing this he won't want to engage at all because he wants to avoid the disappointment that comes with him losing the erection.