If I was a betting man, I'd say his PO has already been made aware of the situation, or will be very soon.
However, why would you want to call his probation officer anyway?
I would think that you would receive a collect call from your boyfriend from wherever he's locked up.
If so, you can ask him.
If not, he probably doesn't want you involved, or he isn't so much locked up as he is shacked up.
Either way, you should give some serious consideration to finding a boyfriend who doesn't continue to be the focus of our criminal justice system.
Just saying,,,,,