Rear end accidents are normally caused by the person following too close.
....Experience does not mean that a rear end accident won't happen. Actually, I presume most rear end accidents are not caused by teenage drivers.
Your son is at fault regardless. Not because he was driving crazy (I can't say that is the case) but it sounds like he was not leaving enough room between him and another car.
Well said.
In this situation, where it's just the bumper (which is usually plastic painted over) that was scratched a tiny bit, I'd ask him to pay 50% of it, and just chalk this one up as a learning experience. I'd be a bit disappointed, but accidents do happen, and to pay $1000 just to smoothen out some paint on a plastic bumper is a bit much. It's not going to start rotting or something. If it was a part of the sheetmetal, it'd be different because it may rust.
This attitude would depend on your (your parents') financial situation, and the son's overall reputation in the past. I guess I'd have my son pay 100% of it if he was generally irresponsible in all situations.
There's no point in the grounding, and if the son is normally careful and responsible for his things, partial payment of the damages still gets the message across withouth completely bankrupting him.