if he went into see his PO and had a smell of alcohol on his breathe, he could be in violation of any "special conditions" that were imposed such as "substain from alcohol and illegal subtances"
however, i dont believe they should have been able to arrest him solely on that. they would have need a "warrant for his arrest" and the PO would have to apply to the parole board for that recall warrant. this is because once someone is released on parole there is a "liberty interest" and therefore, they are entitled to due process.
it is possible that you dont have the whole story and that an "arrest warrant" was out for him and they were going to arrest him when he arrived anyway. or it is also possible that that PO had received the "recall warrant" for some other violation that you are unaware of. anyway, you will want to look into that.
as to the way he was detained, that seems to be a violation of his civil rights under the US Constitution and there could be a basis for a civil lawsuit. so you will want to confer with a local attorney on that matter.
at the revocation hearing, the parole board will evaluate the entire situation - they board can revoked the parole and reinstate the remaining portion of his sentence at its discretion
if you are having a problem with the PD assigned to your son's case, then you should contact the supervising attorney in that office and see what is going on with the matter. keep in mind, that your son will need to give permission for them to talk to you as he has a confidentiality with that attorney representing him