Unfair case, went to trial twice?

My boyfriend recently went to trial for charges in a criminal case. He was found not guilty for Conspiracy Robbery w/Danger weapon, Asault with a deadly weapon/intent to serious injury/kill, and burglary in the first degree. During the trial, the jurors were not able to come up with a verdict for Robbery with a dangerous weapon-individual and kidnapping in the first degree. The District Attorney decided to try his case again. This time he was found guilty for the robbery w/dangerous weapon-individual and kidnapping. But the DA failed to bring in evidence such as the detective working the case and the forensic scientist who found no fingerprints. There was no gun in evidence and I cannot understand why he got convicted of robbery with dangerous weapon when he was found not guilty for conspiracy and asault with deadly weapon. He was sentenced to 25 years for these 2 charges. What should be his next step in order to reduce his sentencing. He lives in Charlotte,NC currently.