Same thing happened to my OH when he broke his kneecap - the GP kept him off work a couple of weeks longer than normal, to ensure that he'd be able to drive safely once he returned to work.
If your son works for an employer and has submitted a sick note, they shouldn't let him work until the GP has issued a Fit Note (the equivalent these days of being signed off) - otherwise their employers' liability insurance doesn't cover them, either, if anything happened to him at work.