I am just reading this post, so I know my response is a little late. In my opinion and this is from experience with a teen ager as well, I don't think your son was intentionally trying to lie or keep this from you. Since the owner of the other vehicle was an adult, your son probably felt like this person was trustworthy and when he "wiped his hand across his car and said there was no damage" your son probably thought this was the end of it.
I am sure if your son had known that the owner was going to contact your insurance company he would have told you. You should definitely talk to your son and let him know that no matter how small an incident is, he should always contact you immediately. Regardless of what the other person says.
As for the the owner contacting your insurance company, I agree with some of the other posts. He needs to be able to prove that your son did damage to his vehicle. And if there was damage, why didn't he call the police at that time. That's normally what people do when their vehicles are hit.
Find out how soon the owner of the other vehicle contacted your insurance company. Had he left from where the incident took place than contacted your insurance company and did he take pictures of the damage while your son was present? All of that has to be taken into consideration.
Investigate this incident as much as you can. Ask your son were there other people around who may have witnessed what took place. If this person's intentions were to file damage claims than the police should have been contacted.
I hope things work out for you.