Well if it was me, I'd ring the garage first and ask for the vehicle's registration number, engine size, make/model etc (check it's taxed as well). Then I'd ring my insurance company from home and get it covered. You MUST tell them that your son will be driving either it or your old one home. You don't want him getting caught for no insurance.
Many companies will do this easily, and will also insure both cars while you dispose of the old one at little or no extra cost.