How do I get my mentally ill father out of prison?

My father was sentenced to 20 years in prison in 2008 for abuse against my mom. The abuse was pretty bad, obviously for him to be given 20 years. While my father certainly was far from a perfect parent, he had a very nice, pleasant side to him for the most part. I've been talking with my mom, and she now believes my father was suffering from some form of mental illness such as Bipolar Disorder or maybe even Dissociative Identity Disorder. Over the years I've talked to my father he has always denied the abuse and has no recollection of physically harming my mom, and I wonder whether he even remembers doing it. He is a good man and very smart and never had a criminal record before this. My mom, the one who testified against him now feels bad all these years later and thinks the judge gave him too harsh a sentence and 7 1/2 years in prison has been long enough. Plus the fact that she thinks he was mentally ill when he committed the crimes. With my mom's help can we help him get out?