So, basically, here. The lane markings, if unchanged, are clear about what a driver in the right-turn lane is expected to do.

We can only make an educated guess, but I expect that the other driver's insurance company is taking the position that your son saw or should have seen the vehicle in the right-turn lane before proceeding and that the accident was caused at least in part by his failure to observe and yield, and would have occurred regardless of whether the other driver turned or went straight.

Perhaps the other driver confirmed your son's version of events to the police, but if he did not there is a possibility that he is giving a different version to the insurance companies -- such as, he was intending to turn right into Home Depot but straightened out and tried to get behind your son's vehicle to try to avoid a collision after your son cut him off.