Encourage him to crank it up, with the windows up, and down, and stand in the driveway and listen for rattles.
A big offender can be the license plate. It's always amusing to hear a car where the owner thinks its cool for the whole block to hear his thumpin'bass, but really what we hear is the buzzing of a license plate clattering against the trunk lid or plate frame.
Also encourage him to follow a rule I set for my daughter..... keep the volume low when you are close to home. All trips will start and stop from home. People know you (or your car). Once you're away from home, feel free to annoy people. Statistically, you won't encounter those people as often.
My rule was, if I ever hear your stereo, it's too loud too close to home.