He needs to hire a lawyer to look at the offense history in which he pled guilty. Certain offenses are mandatory registration and some are not. So, depending on the evaluation, the lawyer can help with the communication with PO. Obviously, it may be better to move back to the jurisdiction in which the PO is more favorable.
He needs to talk to his original attorney. If you are required to register as a sex offender, the the judge normally would say that at sentencing. Registering is a serious matter, so get counsel and figure it out ASAP.