Bhg481, I was typing this late and night after a few glasses of wine so not entirely clear! What I was trying to say is the sons insurers would be obliged to give third party cover when someone else was driving his car, if that person had no other insurance. In this case the person driving, ie the garage employee, should be insured under the garage's motor trade policy.
Dancermo, get your son to call his insurers to check out the situation but as he wasn't driving the car at the time of the damage I am sure they will tell you to claim through the garage on their policy.