As I don't have full information, this is only speculation, though I wouldn't be surprised if this person is trying to make a claim out of nothing here. He may've thought that he got his so now he can say that several days later he has begun experiencing back pain and is now wanting to claim significant money from your insurance. That being said, the facts will speak for themselves for the most part, so with no damage to either car, it will be difficult to say that the cars were going at significant speeds such that the collision would cause him injury.
In any event, at this point, there's not much you can do except sit back and hope he doesn't put a claim through given the lack of actual damage, and assuming he doesn't your insurance company won't ever get involved.
Hopefully that is of assistance.
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