Good advice from Flow4, I think!
Yes, I'd be worried too, but I like your approach, gently chipping away at your own son, who sounds reasonable, offering to take them etc. I think you might well manage to wear him down, eventually. And in return for him respecting your wishes, you could offer to give him driving lessons? (Bribery works well in my family ...)
If the worst comes to the worst, in your eyes, and they do go in M's car, try not to worry, because most people, even young men, don't want to die, and there is a self-preservation instinct in there. Ask your son to guarantee that he will not get in the car with M if he has had even one drink.
Speaking as someone whose son did have an accident on a motorway aged 18, btw. No-one was hurt, but the car was a write off. My youngest is just about to take his test, but I feel (slightly) safer knowing he will be the one to drive his friends around.