^^ He's already got a mirror.
I'd just get used to checking it more frequently. On some stretches I ride, I can't hear a bike about to pass because of the noise of the freeway traffic just on the other side of the barriers.
I've pretty much given up on the sense of hearing as a safety tool. Riding a bike around town or at a decent speed is louder than people seem to realize. Think of this, though -- when you have to raise your voice just to talk to the cyclist next to you, it's loud.