Then he shouldn't worry.
As long as he can document his calls to the PO officer and has talked to the PO officer's supervisor - he's OK.
I don't want to understate that probation and parole violations
are serious offenses that often result in harsh penalties such as community service, heavy fines, extended jail time, and more. The consequences associated with a parole or probation violation usually depends on a number of factors, including prior convictions and/or parole violations.
But, since he's made every effort and can document or itemize his attempts to contact his PO officer and has talked to his PO officer's supervisor - if the PO officer would take the extreme wrongful position and violate him he would go before the presiding judge who he could explain his multitude of efforts and no responses.
Clearly, under the circumstances the judge would no nothing and problably reprimand or scold the PO officer.
So, I would just inform your son to make sure he continues to attempt to contact the PO officer and that's all he can do.
But he shouldn't worry about his PO violating his probation under the circumstances.