What legal steps do we take to get another public defender appointed to him?

My son’s public defender has not done a good job representing him from the beginning. My son has been in jail fighting his case for 4 months. He never talked to my son though my son always tries. The public defender talked to me a little bit. We have wanted to get another public defender since the beginning but we were told that the judge usually denies that kind of request, so we never asked. Then just today, I requested an appointment with his public defender because there are so many errors from the DA and cops that I believe we can get a dismissal. But when I called his PD’s office today, his secretary told me that we couldn’t talk to his PD for 17 days because he is busy in a trial. I think that is grounds right there to be granted another PD. Time is running out. Don’t you think he should get to have another attorney appointed to him, since this one is now refusing him at this crucial time? He has a court date in 6 days and we need to consult with an attorney. What should we or what can we do?