Whether the attorney is court appointed or not, they are not allowed to withold any evidence. If you believe an ethical violation has occured, contact the State Bar and file a complaint. Thank you and I wish you the best of luck!
If you think an attorney is withholding evidence that you think needs to be brought up, then you have a few options. Your nephew could simply ask the court for a new attorney if he believes this attorney is not presenting the information he wants presented.
Also you could attempt to talk to the prosecutor and provide your undisclosed evidence to the prosecutor on your own.
Also you could report the attorney to the state bar.
Further many counties have basically a "complaint department" with oversight of court appointed lawyers. You could file a letter with the people that oversee the work that court appointed lawyers do in you county and possibly they would open an investigation into the complaint against the attorney.
I am not suggesting any of these methods would work to draw attention to your situation, but if the attorney won't bring out the evidence and you believe it is important to your nephew you may have to step around the attorney you have.