I'm sorry I can't help. You've already done all the things I would have suggested. This one is a new one on me too. Has he been groomed lately? Sometimes mine will scoot a little after having his little bum shaved. But I've never had them want me to rub it though..
Is he intact? He may think it is a little love ritual?? I don't know.. grasping for straws..
Hope you get to the "bottom" of things quickly..