You are unlikely to find an attorney to take this case pro bono. He may be able to get a court appointed attorney on appeal. You will have to contact the court to find out if this is an option. The only other option if you cannot afford to hire an attorney is to contact the Innocence Project. They are overwhelmed with requests but will review the case to see if they can help him. They do great work.
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Please be prepared to pay a hefty lawyer fee in this matter. Many lawyers will not agree to become the 4th lawyer on a criminal case. If they do, they typically want to be well compensated and want their fee up-front. There may also be significant investigation and expert expenses. I don't want to discourage you, but to prepare you for some sobering realities. Good luck.
In addition to the comments above, I would add that you do not want "help to file a writ" if that means you intend on doing so without counsel, or worse, with the help of a "writ writer" inmate. We do writs in our practice, and I cannot tell you how sad it is to see people coming in wanting to file a writ correctly after someone else has already filed one. This is usually a one bite at the apple type of solution. Please, please make sure you get the assistance of a good lawyer before filing any writ. It will definitely cost, but it is better than ruining any chance he may have.