Criminal Law Questions? Ask a Criminal Lawyer.
To apply for a pardon, a letter will not do. You must (actually your nephew must) fill out an application for clemency, which can only be obtained by requesting it in writing to the governor's office. The inmate must also send a notice the prosecutor's office in the county he was convicted in.
See instructions here: http://www.cdcr.ca.gov/BOPH/docs/apply_for_pardon.pdf
A sentence modification is all together different. He would have to Petition the court that sentenced him directly. Generally, his sentence can only be modified if he is out on probation or parole, his actual prison time can't be reduced.