Paying his late assessments can't hurt and it may help, but we can't promise you that his probation officer or sentencing judge will forgive his transgressions.
If he's being held in jail and nobody posts bond, he can expect to remain in jail until the issue is resolved. Generally speaking, he can expect to end up back on probation, perhaps after serving some jail time, and again being expected to pay his fines. We don't know the facts of his case, so it would make sense for him to consult a lawyer with the details.

