Very unlikely that anything can be accomplished at this late date, and your posting does not suggest that your son had a very strong case to begin with, but if there is any avenue of relief it is probably going to be through a post-conviction petition. Not too many attorneys care to handle those because they are difficult, specialized, and frustrating, but that is where you need to begin, by finding an attorney who is willing to look into it and see whether such a petition has any possibility of success.
I agree hire a post conviction attorney, or at least talk with one. Good luck.
This is not intended as legal advice. No attorney / client relationship exists because of this response.
I concur with my Avvo colleagues, a review by an experienced post-conviction attorney is good next step.
Of course, every answer or response is based on the information provided in the question asked and requires a much more complete context than is available in this public forum. This answer/response should NOT be relied upon to make any legal decisions. Seek the advice of an experienced Federal and/or state criminal defense attorney in your jurisdiction BEFORE you say or do anything.