Motorway merging protocol, with your son in front, places responsibility for the 4wd on the motorway to either slow down or change lanes. The driver of the 4wd was clearly not reading merging traffic in front of him and traffic in his lane in front of him if he rear ended your son when he had to slow down for the traffic in front of him.
I cant think of an excuse to allow you to run into a vehicle in front of you.
He needs to make a claim against the 4wd's insurance company ... it will probably take some time to get paid though ... its a shame he didnt have comprehnsive as his insurance company would then have done the leg work with the other insurance company.
