If the sole basis of the appeal was that the judge gave the incorrect sentence, and the judge has now corrected the sentence by revising the sentence to the correct term agreed to in the plea deal (a revision of a criminal sentence normally must be made in court with the defendant present), then your appeal is now moot. It will likely be dismissed by the appeals court. A judge retains revisory power to correct a clerical mistake in a judgment (if that was what the mistake was) at any time.
The answer to your question is answered by Maryland Rule 4-345. Since your case was in the circuit court, the judge had revosiry power over the sentence, and thus did nothing wrong. Unfortunately, your appeal will likely be dismissed, unless you had some basis for the appeal other than that the sentence exceeded the plea agreement. Having said that, it may be worth consultation with some criminal lawyers just to see if there is any other appealable issue.
If what you are referring to is the motion to modify, then it must be filed within 90 days of the Judge's original ruling in the case. That motion can be held sub curiae by the court until your son has sufficiently complied with his sentence/probation and the Judge feels he wishes to modify the sentence. You should immediately seek counsel to help you file the motion, so that it will have been filed if the Judge wants to modify your son's sentence down the road.
The court may fix its own mistake. Your son has 90 days from sentencing to ask for modification by the original judge.
The court's mistake does not generate an opportunity to try to renegotiatie the plea bargain.
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