It's up to the judge, ultimately. It sounds as though the prosecutor wants the total fine paid up at the time of court. Is that correct?
In Georgia, any person found guilty of a misdemeanor for the first time, who was on the date that the misdemeanor was committed between the ages of 16 and 18 years old, shall be punished by a fine not to exceed $1,000.00 or confined exclusively under the jurisdiction of the Board of Corrections for a period not to exceed 12 months.
What this means is that the judge can send him to jail. Probation revocations work by brining the original sentence current (with credit for time served and fines paid). The prosecutor (with the advice of the probation officer) will make a recommendation to the judge, who will have the authority (based on a preponderence of the evidence standard, rather than beyond a reasonable doubt) to make the determination. Many times this is a rubber stamp of what the prosecutor (and probation officer) request.
Your son should talk to the prosecutor if you are not happy with the terms of the request, but all things considered, it sounds like a good deal. I'd go with it. The judge can order him to go to jail for the entire amount required under law, and require the entire fee. Just having to pay the fine is preferable to jail, in my opinion.
Good luck to you and your son!