You should hire an attorney for him immediately. If his previous drug charge was a felony, that means he's not allowed to possess firearms. So, he's looking at an unlawful possession of a firearm charge and possibly malicious mischief for the graffiti, in addition to violating his probation.
You should advise him not to talk to the police or anyone else about this incident until he has talked to an attorney. He either needs to hire one or have the court appoint one if he can't afford to hire.
You can help him get an attorney, but most of what needs to happen involves your son making some serious life changes. You can't make those decisions and changes for him, but you can provide moral support as he makes the changes himself.
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