Deaf guy I know uses a flashing light sort of dealy.
Punch in "flashing light doorbell" or "deaf doorbell" on Google and you'll find all sorts of devices for sale as well as instructions for converting standard doorbells with louder signals and strobes.
On a related note, I know a nearly deaf guy who has hearing aids that connect to his iPhone with Bluetooth. The phone vibrates and strobes the camera flash when he gets a call.
Anyway, there are also gadgets called "Smart Doorbells" (Skybell, Ring, and August are some brand names) that have a camera and a motion sensor and notify you via your phone in the same way when there's someone at the door. Even if you're somewhere else, you can see who's at the door, and some units have a microphone and a speaker that let you communicate to the visitor.
If said guy is not quite completely deaf and normally carries a smart phone, a smart doorbell could be a good option.