I’ve got a satnav which beeps at you when you exceed the speed limit, that might work for him. Sometimes you need it, because of variable speed limits. I drove down to Swansea this morning, and there were 17 different speed limits on a six mile stretch - I was counting as I couldn’t believe how many there were the last time I drove that road - it would be very easy to get caught in those circumstances however good a driver you are. Another alternative is that you refuse to go in the car with him, and take over the driving yourself, then you won’t need to worry about him speeding, if you think he drives dangerously.