wow, he's stubborn, isn't he. You might put it in terms that imply that he might be AFRAID to ride by himself? That might motivate him to prove you wrong.
Bring him over here, I'll thwack him on the head.
I second that. That would drive me CRAZY. Too much togetherness. I feel claustrophobic just listening. YOU might have to whack him over the head, go on strike, or something. Reasoning does not seem to be working, does it?