You can try contacting the State Bar of Arizona and see if they have resources to help your son.
To add to the other answer, your son could also ask the judge to assign another lawyer. Doesn't mean the judge will, but you can make the request. Make sure you have actual reasons to give the judge, not just "I don't like him." Document when your son has tried to contact him and hasn't been able to etc.
Lastly, the prosecutor will have access to see what your son's juvenile record is. It can't be used against him in court or to aggravate his sentence, but the prosecutor may use it to decide what kind of kid your son is and tailor his plea offer accordingly.