Make sure he has no access to the car key.
One incident:
A friend of mine has a daughter (elder) and a son. The boy was spoiled. He
got whatever he wanted and he loved sitting on the driver's seat. One day
in their garage, the boy and the girl were inside the Van and the mom went
upstairs to get something. Then she heard a scream and a BANG! She looked
out of the window and saw her Nissan Van crashed her cross-street neighbor's
fence!
The car key was left in the car and the boy somehow put it into the keyhole
and then put the car on backup mode. The hand brake was off, so the car
just slid out of the garage. The daughter screamed in horror! Luckily there
was no one outside the house in the path of the moving van. Both kids were
Okay.
I think that van has a design problem. For Toyota vehicles, without starting the engine, the gear shift does not work. Somehow the Nissan van did not have that
safety feature. It may have it now, I hope.
