I'm no expert, but I would think it would be HIS problem as to any sort of reporting cutoff date, since he's the one wanting to report it.  Inform your insurance agent, explaining that you didn't think getting them involved was appropriate, but that you are doing your due diligence.  Don't make any official claims, but inform them of the situation.

That's what I would do.  My insurance guy is pretty great about telling me what I need to do in any circumstance.  Talk to yours, and they should do the same.  That's part of the service you pay them for.

Eek, technically (although I know you probably don't want to hear this), if you (or your minor son) is responsible for damage to another's property (an automobile in this case), it is your responsibility to notify your insurance company (so that if there is a claim you have provided proper notification). That is required in most auto insurance policies (otherwise the insurance company can may be able to deny indemnity, and you might be on your own). A person who is damaged has a legal statute of limitations to demand to be made whole again (I'm not a lawyer, but this statue might be a year---perhaps the lawyers out there know for sure).