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Your son should probably say nothing without an attorney. You didn't say how old your son is, but it is probably a juvenile matter. But, he could say the wrong thing and get himself in trouble. It is better if he keeps silent and talks to an attorney.
When police called, I was telling the police that my son was in the car when this happened, and he's not aware that this other kid would light a fire. The police said they will/might call him as witness. I don't know why the police kept saying my son is lying.
You should not have said anything to the police and they will keep trying to intimidate him and you. Don't let him say anything.. If they call him as a witness, he needs to refuse to answer. He needs a lawyer if at all possible.