How to get a sentence reduced

My son accepted a plea and received a charge of aggravated robbery and received a 25 year sentence for a first offense. There were four defendents in this case. Shooter (gang member) received life, 2nd defendant with prior felony convictions (gang member) received 45 years, 3rd defendant (my son not a gang member) with no prior felonies received 25 years. The 4th defendant with two prior felonies and gang related who testified (in another case) and who contacted the other two gang members in my sons original case received 5 years. How can I (we) my son get his sentence reduced after the 30 days from sentencing has expired?