If the son was driving at the time and the Merc was a company car it could well be that the son isn't insured hence the reluctance of the Merc people to go the insurance route.
Certainly when I used to have a company car only me and the wife were insured not my children.
When I was a wee-wee-lad, many, many years ago I took my Dads company car for a midnight cruise and crashed it on the Hanger Lane gyratory roundabout. Absolutely **** myself, had friends in the car and I couldn't face my Dad so went to a mutual friends house, got her to ring my Dad up at 2am to tell him the bad news. To his credit he never flipped and reminded me that I was an idiot and that I wasn't insured. Luckily he was quite high up in the company he worked for, pulled some strings and got me insured, never heard anymore about it but it did teach me an important lesson, namely it's a bad idea to exit the gyratory from the inside lane across the path of a car that isn't indicating.