They're offering $500. A paltry sum, in my opinion. I would have my son counter their offer, of course.
Sorry, Zero injury = Zero dollars.
$500 is a gift.
However, I'll give you the claim rep's point of view.
I have over 100 claims open at any one time. To keep up a good performance review (and to keep my head above water) I have to close as many as I open. Otherwise I have a back log as new claims come in every day.
Small claims can be settled quick, large claims take longer. I don't want small claims to linger so I'm willing to throw a little money at them to get a release and get them gone.
With that in mind, I suggest you have your son call the person who made that offer and say "Your driver's negligence gave me quite a jolt and inconvenienced me. Give me $750 and you close the claim today. Otherwise I'll just have to wait and see if I develop any aches and pains.
While this has worked for me as a claimant and I would be willing to pay another $250 as a claim rep, there is also the risk that the claim rep will just withdraw the offer completely and close the file.
Your son has no leverage without an injury so he has to be careful about how greedy he gets.
Remember Aesop's fable about the dog and the bone. a dog carrying a bone over a bridge looks down into the water and sees its own reflection. Taking it for another dog carrying a better bone, the dog opens its mouth to bark at the “other” and in doing so drops its own bone into the river.
Your son could lose his $500 bone in the quest for a much bigger bone.