@Karmacamelia then get a dashcam that also records the interior of the car so you can see what he is doing and check the footage. 6 points on his license if he is caught using his phone and make sure he knows that doing it whilst stationary traffic is also illegal no matter how many people argue it shouldn't be. We have a Vifo dash cam, really good quality footage. Plugged into the usb and comes on automatically when we start the car.
I would look at dash cam videos with him on youtube, Ruby Dashcam and Dashflix are our go to ones and there are others. One of the main crashes you see are exiting roundabouts but not from the left lane so the car in the middle/right lane totals the car in the left lane who is continuing round the roundabout. This has case law and is called Grace and Tanner, both cars are at fault. So for your son, being aware of going straight on at a roundabout using the left lane, position yourself so you are not immediately next to the car to your right, check mirrors all the time. In this house we say "Constant vigilance" which is a quote from a book.
I agree you have to play bad cop but I never had. My two sons are incredibly sensible which may be because 2 school mates were in a horrific car accident due to driver error. The school really drove it home how easily it is to die but also survive, need 24 hour care and never walk again. One did die, the other is the 24 hour care part.
If you are able to take them in your car once the driving instructor says it is safe then get him to narrate his thoughts as he drives, we also narrated it too so they could see what we looked for. I did what we called finessing which was positioning, lane discipline sort of stuff. Dh took him out all over the place, followed a sat nav and he probably did about 600 miles of private driving with us whilst a learner.
My car is all singing and dancing, lane discipline, blind spot mirror warning, collision avoidance, all sorts of sensors for reversing and at the front of the car etc. I chose this deliberately because they would be driving my mid sized car and not Dh's car.