What I believe he has to do is file a 17B motion to get the felony reduced to a misdemeanor than file to have the misdemeanor expunged. Both can be done at the same time.
The DA's office and the Probation department will have to be served some papers. He really does need to get a lawyer that is versed in gun law and restoration of ons rights. I don't think this is something you want to try yourself.