It will be very difficult to find someone to take this on, especially pro bono. Since he doesn't actually have a case pending, a public defender is out of the question. In this situation, you are attempting to find or create an exception to a State statute which is very challenging, very time consuming, and very unlikely to be successful. Most attorneys take on pro bono cases based upon personal convictions, so you'd need to start with some sort of organization that has a mission of changing sex offender laws. I don't know of such an organization. I wish you luck, because I have seen cases similar to what you describe and know that the draconian outcome doesn't always fit the circumstances.