Arizona rule 31.3 requires that a notice of appeal be filed within 20 days of the date that the sentence is pronounced. So if your son was sentenced more than 20 days ago, it would generally too late for him to file a notice of appeal at this time and most of the grounds for an appeal would be closed to him.
It's also very difficult to get a plea reversed after one is sentenced on it. That is because the judge asks questions on the record to try to make that plea airtight to make sure the defendant knew what he was doing when he took the plea. Those questions and your grandson's answers are on the court record and could be used against him if he tried to take his plea back.
Furthermore, in order to get a plea a defendant has to waive his right to challenge the evidence against him and confront witnesses. If he wanted to have his day in court, present his evidence and have his lawyer cross examine the state's witnesses and get the whole story out there, he'd have had to choose trial.
But if he was convicted after trial, he would not get the same deal he accepted. He would get much more time because a jury would have convicted him beyond a reasonable doubt.
There may still be things that he can do even if it's too late for most types of appeals. You could contact a post-conviction attorney on his behalf and see what remedies might still be available. But understand that even if he did get his plea back, he will have to face the original charges and try the case, and that's exactly what he didn't want to do in the first place.