Ask him to do some 21' foot drills with you.
He holsters an unloaded gun after both you and him verify its unloaded.
Set up a target about 4' from him and have him face it.
If the target is 12 o' clock, you start out at about the 9 o clock position if he's right handed, 3 if he's left handed, 21' away from him. He should be able to just see you out of his peripheral vision. You are on the opposite side of his draw and he should never sweep you while presenting to dry fire at the target. Once you get used to this drill, and with a little equipment, you can safely step it up and do a head on charge while he tries to dry fire at you, but we'll get to that later in the post.
With no signal, at some point you rush him. Your goal is to touch him before he can unholster and dry fire at the target. His goal is, of course, to rack his pistol and "shoot" the target before you touch him.
If you're willing to invest $15 and are reasonably comfortable with charging him straight on without crashing into him and both of you toppling to the ground, buy a training barrel and let him attempt to draw and rack before you're on top of him while you are rushing him straight on.
TB002 Training Barrel : TRAINING BARREL | Brownells
That's a training barrel. Lets the slide function, etc, but obviously no way for a round to be fired as its just a barrel shaped chunk of yellow plastic.
A good friend of mine, who's also an excellent trainer, set me up with that drill (among others) vs a chalk edged rubber knife. Its a humbling experience and also an eye opener for your equipment and set up for quick draw, firing from the retention position, etc.
Especially if you are CC. It adds time to the drill on the shooter.