The good news is, you probably don't have to hit him in the head.
Of course an open wound will make you more prone to infection, although if he isn't carrying anything he can't give it to you, no matter how clumsily he goes at it. I suggest putting him on notice that you intend to stop him the next time it hurts and show him some alternative moves. You don't have to put on a show — just ask him to start with the gentlest, most lubed-up touch he can manage and move up from there till you say "when." If he's actually concerned about hurting you and wants to do better, he will be motivated to pay attention. If he isn't ... well, what are you doing there?