It's difficult to say who much, if any time, he will do in jail. This is because, if it's a county sentence (being supervised by county parole) the judge will make that determination. It sounds like he only has one technical violation, so the PO may even chose not to detain him. If he did have a hot urine, as I suspect that's why he ducked the test, he should talk to his PO as soon as possible about going to rehab, either inpatient or outpatient.
He should seek counsel if detained.
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Generally, a person will not be sent to prison for a violation such as the one that you briefly described. This is not true if there has been a history of probation violations, failures to provide urine samples or "dirty" urine samples (UA). Most probation departments treat a missed UA as a failed UA. This is very different than an actual dirty UA...a urine sample that detects alcohol or drugs. If this is the only basis for the probation violation, I would find it hard to believe he would be sent to prison or serve any lengthy jail sentence. Obviously, your question was very brief and did not provide many details, so I cannot be certain that my conclusions and experience with these matters applies to your son's case.