Are you sure that the PO violated him on it? If so, the penalties can range depending on the PO's input, the Judge, the terms of sentencing of your son, etc. He should definitely have an attorney represent him on this matter.
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Is roommate willing to testify that it was his? A lawyer will help you put together defenses and information surrounding this PO visit.
Curt Harrington Patent & Tax Law Attorney Certified Tax Specialist by the California Board of Legal Specialization PATENTAX.COM This communication is general information and not legal advice, and does not create an attorney-client relationship. This communication should not be relied upon as any type of legal advice. Please note that no attorney-client relationship exists between the sender and the recipient of this message in the absence of either (1) a signed fee contract and (2) remission of an agreed-upon retainer. Absent such an agreement and retainer, I am not engaged by you as an attorney, nor is any other member of my law firm.
I agree with my colleagues that a) it depends, and b) you need a lawyer if there is a reported violation of probation.
Consequences can range from extending probation, fines, classes, to jail time, or the maximum of the penalty for which your son was put on probation. For example, for Felony DUI, that would be three years in prison.