Yes, probation transfers are possible, but they require cooperation from multiple parties and a compelling case for why the transfer serves the interests of successful supervision.
The process is called an interstate or intercounty probation transfer, and it works like this. Your boyfriend would need to formally request the transfer through his San Diego probation officer. That request gets reviewed by the sentencing judge in San Diego, who has to agree that transferring supervision is appropriate. The receiving jurisdiction, in this case Kern County where Bakersfield is located, also has to agree to accept supervision of the case. Both sides have to say yes before anything moves.
The argument that actually works in these situations is straightforward and practical. Stable housing and legitimate employment in Bakersfield are the two strongest cards he can play. Probation departments want people to succeed, and success is harder when someone is forced to maintain ties to a jurisdiction where they do not live, work, or have a support network. A San Diego judge who sees that transferring supervision to Bakersfield gives him a better foundation for completing probation successfully is more likely to approve it than one who sees no compelling reason to deviate from the original order.
The weakest version of this request is showing up without anything confirmed. The strongest version comes with a verified address, a job offer or employment already in place, and family support documented in Bakersfield. The more concrete the plan looks on paper, the easier it is for both jurisdictions to say yes.
His attorney or the San Diego probation department can walk him through the formal transfer application process and timeline.