If he did not appeal his sentence within 30 days, it became final. To overturn it, he needs to file a post-conviction 11.07 state writ. He must allege constitutional violations such as ineffective assistance of counsel, new evidence, or the prosecutor's failure to turn over Brady evidence. He should hire a post-conviction appellate lawyer that handles post-conviction 11.07 state writs as these type of appeals are complex. If his appeal is successful, his conviction is reversed and he can plea bargain or go to trial. Good luck.
My answer is for informational purposes.
Not very likely. He's entered a plea. He would to show some defect in the Porifera and that's not very likely.
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Unfortunately the fact that he plead wipes away his right to appeal unless he did not waive the right to appeal during his plea. You should seek out an appellate attorney in your area to review this situation, but unfortunately I am afraid he is going to be stuck with the conviction due to the violation.