I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear that this happened.
A summons does need to be served personally or left with someone of suitable age and discretion at the residence (like a spouse or a roommate) AND mailed. A summons hung on the door is not properly served and he could move to dismiss the case on that basis alone. However, it's important that he reply now that he knows about the case, because it's difficult to get a default judgment removed when the defendant had actual knowledge of the case.
A citation is not required to sue someone for damages. Police arrive at the scene and take reports, but they're not official arbitors of what happened. A police officer's determination is not legally binding. Any injured party has a right to go to court and prove that someone owes them money. But that doesn't mean they win. Your son needs to read through his contract with the company carefully, but I can't think of any way he would be liable for getting hit from behind. He should be able to defend on the basis that he did not cause the accident.
If the contract doesn't require your son to reimburse the insurance company, the claim is too late - there's a three year statute of limitations on personal injury. And if it does, he can dispute on the grounds that the clause violates public policy. There are many reasons it's bad practice for an employer to require an employee to indemnify them from loss, and this is one of them.
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