You can research to see if you can find any inmate advocacy non-profit groups or clinics at law schools. That's the only place you are likely to find anyone working for free.
Did he have a high-quality trial (defense) attorney, private or public defender? If he says he is factually innocent...didn't do it...contact the "Innocence Project" (via internet). If he can't get advice through them he is probably stuck.
Contact the innocence project or the ACLU. You are not going to find a free attorney in the private sector to us a law practice is a business and representing clients for free would be like a super market giving away groceries.
Consumers can use this platform to pose legal questions to real lawyers and receive free insights.
Participating legal professionals get the opportunity to speak directly with people who may need their services, as well as enhance their standing in the Lawyers.com community.