I think you're asking whether he could move to another COUNTRY, not county, right?
If so, after probation/parole is over, a sex registrant can move wherever they want.
The question is whether you can come back.
If you move somewhere with sex registration, you must continue to register there and be able to prove you did so upon return to CA. If you move somewhere without sex registration, you may have to provide proof of where you resided when you return to CA, and that no sex registration was possible.
There are also some states in the US that do not have sex registration.
It depends on what happens with his case. He should talk to his attorney about the consequences if he loses, because every situation is unique.
Generally, if he wants to move to a different county while on formal probation, he would need to get his probation transferred which requires approval. If he's not on probation, but is required to register as a sex offender, he can move as long as he follows the restrictions on his residency (for example, one restriction might be that he can't live within 2000 feet of a school) and immediately registers (within five days) with the local police department in his new city/county of residence.