You can check with the Plumas County Jail for their procedure for getting married if you want to try to do it before he is shipped out, but people who are sentenced to state prison can be picked up at any time. The Department of Corrections & Rehabilitation (CDCR) tries to keep people from finding out when prisoners are being transported to avoid escape attempts.
Once he gets picked up, he will be transported to a CDCR reception center, probably the one in Susanville. It is unlikely he will be allowed to get married while he is in reception. It can take several months for him to be placed in the facility where he will actually do his time.
When he permanently placed, he will need to get the warden's permission to get married. The warden will consider his classification (based on severity of his crime, violence and whether a weapon was used) and his behavior to date. His prison counselor can help get that process moving.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like Avvo, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.
I agree with Attorney Marshall's answer and would add one more possibility. If his attorney can perform the ceremony (i.e. he is a licensed minister), he might be able to bring you in with the witnesses to the interview room and do the ceremony and bingo, you're married! I've done a version of this a few times for clients in the past. This is done while he is still at the jail, not the prison. You're attorney has to have access to do the ceremony.
Good luck with your situation.