OK, your son needs to contact a criminal lawyer asap.
Normally I would say that he is under no obligation to return the call of the police officer. They will want to talk to him about it, and anything he says can be used against him. They might want for him to come in for an interview, same thing. This is why one shouldn't talk to the police; your son is under no obligation to help the police gather evidence against him to lay charges.
If they are going to lay a charge, then they'll lay the charge. He can't talk his way out of it, unless it's a case of mistaken identity or he has a rock solid alibi.
If your son gets a criminal lawyer to call the police for him, then that's much better. Your son won't say or do anything that can be used against him.
If your son wants to call the police then he can. But he must not say anything about what happened, especially if it turns out he actually was there and the police have the right man. Remember, he can't talk his way out of a charge if the charge is coming, and if the police actually have two witnesses that are good then your son will be charged regardless of what he says to the police.
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