Does your son have a dash cam in his car? If not, you may struggle because the the insurers may see it as 50/50 liability. It also depends on the road position of your son's vehicle. If he was at the junction behind the double hatched white line, the responsibility is on the other party to ensure the road that he's just emerged from is safe to reverse back into. It would then be 100 per cent his fault. If your son has crossed the double hatched white line, he now has an equal responsibility to ensure his path is safe to proceed, and should not proceed until he has checked both directions completely.

It does sound as if your son may be a victim of a "vexatious" claim, possibly fraudulent. Make your insurer aware of this and ask them to contact the other party's insurer to request they provide their client's explanation of events. If he advised he was reversing, it should be clear to his insurers he was at fault. If has not, you have a battle on your hands. It would then be up to your son to prove his version of events. I suggest you draw a detailed plan for your insurer to explain what happened to assist them.