When I leave my transit on anything but a flat road I always put it in gear or walk back to it cos its bugging me that I forgot
I just don't t trust the brakes on it since the hill start is on it
Brake systems need to be simple
The more gadgets you put in the more there is to fail

I don't blame your lad
Fooling car shouldnt just roll off when it feels like it
Certainly not a nice lesson to learn