My nephew was arrested for a crime he did not do, he sold a motorcycle on craigs list and it was involved in a police chase and other crimes, the boy came to purchase the bike he had not plate so my nephew allowed him to use his to get home, what a big mistake, they have a picture of the boy from face book bragging about his new ride and a lot of other evidence, but the court appointed attorney failed to show these things to the judge and they are gonna sentence him with the crime, it is so wrong, he needs help, he lives in Oxford and the crimes are in Detroit, were he never goes, He takes care of his 85 year old grandfather, and his mother who has stage four bone cancer, he is desperately needed at home and so innocent, we haven't the funds to hire an attorney. I will prove all I say.
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.