If your son wants his bike back he will pay the impound fee to get it back. Whether or not your son's friends would be legally liable for part of the fee because he too was riding the bike - good question. To find that answer would take substantive legal research which I would only undertake if retained to do so.
According to sections 511(b) and (c) of the NYS Vehicle and Traffic Law, which deal with the seizure of vehicles due to their unlawful unlicensed operation, the registered owner of the vehicle shall be responsible for such payment provided, however, that if he was not the operator at the time of the offense he shall have a cause of action against such operator to recover such costs.
Most likely your son with a potential against the driver, and you should go to small claims.
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My response is for educational purposes and does not constitute legal advice nor creates an attorney client relationship which requires all the details and a personal conference.