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Are you asking whether he must go to rehab if probation wants him to go to rehab? The answer is yes, he must go to rehab and successfully complete it, or he can be violated on probation.
He is on probation because he's a sentenced criminal offender. Probation is a sentence in lieu of jail. That means the only thing that is keeping him at liberty is his compliance with what his probation officer wants. As a convicted offender, the only right he has when he doesn't like what probation wants for him would be to get before the judge and try to get the judge to overrule probation.
His sentencing judge has the power to keep him out of rehab even if probation wants him there. But judges usually don't go against probation unless they find probation's decision to be unreasonable or arbitrary. Then they will step in to correct an injustice.
So your son can contact his lawyer, and if he has a compelling argument as to why he doesn't need rehab and what he can be doing instead of rehab that would be better for him at this time, his lawyer may be able to persuade the judge to let him get some other type of program that isn't in-patient.
If this isn't the question you were asking, please clarify by a reply to this question thread, and I can modify my answer.