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Your son is not in a good position, unfortunately, because probation is a sentence given in lieu of jail. He needs to speak to his probation officer about the possibility of getting his case transferred Stateside. If probation says no, then probation can help him get the case back before the sentencing judge to see if the judge will allow it.
Frankly, they don't have to allow it. Back when your son took this plea, he agreed to its terms, and he was probably told how much jail he could do if he didn't successfully complete probation. That's what he's risking if he doesn't complete probation.
If the court says no to a transfer, he can ask for a larger fine or for a lot of community service which he could finish in a couple of weeks. If the court says no to that, his option would be to return to the mainland, whereupon a warrant would issue for his arrest in Guam, or to look for a new job in Guam that would allow him to afford to stay there for long enough to be eligible for an early release from probation. (He can apply for that at the halfway point).
He should first consider visiting the local public defender's office. Maybe they will agree to step in since he's not working and in a financial and legal jam.