It would be a request of the judge at sentencing. Talk to his attorney.
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Generally the Court will allow someone convicted of a crime to make restitution payments during the time they are on probation. Bring pay stubs and proof that a payment plan would be appropriate for your son to his sentencing. As long as he makes regular payments as directed by the court he should be alright.
This will be vary case-to-case and court-to-court. Money is often the sticking point in criminal cases so I understand your frustration. The bottom line is that some judges allow payment plans and others don't. I am not familiar with the procedures in Saginaw but I can say in Macomb, where I frequently practice, most of the courts will allow some time to get everything paid down. I do not, however, know the specifics of your case. Sometimes, where an exceptional deal is offered, it will be conditioned on immediate payment of restitution. Perhaps getting a hearing before the judge will help to bring this issue better into focus for you. Good luck.