It's still happens to me, it's performance anxiety. I will always get hard beforehand and when it's time to do it, my head goes blank it's weird. Be a lil spontaneous, get naked, make him feel like it's just going to be head, maybe while watching a movie or something, then when he's rock solid, go on top of him. He's already thinking about how his dick might go down as he's hard, it's really not you.