Legal Question in DUI Law in California

It is likely he could, but we would need to see the documents from the court. Worse case scenario we could petition the court to allow his probation take place in MO.

If he comes off 'formal' probation after the jail stint, he could probably leave without penalty or need for permission. Informal probation does not impose 'stay in CA' orders normally. If he is still bound by formal 'don't leave' probation, then he'll have to file an appropriate motion in the convicting court, and provide support for his request. Lots of permissions and details are involved. If serious about hiring counsel to help, and if the court is in SoCal, feel free to contact me.

He is likely just on informal court probation, in which case he can leave the state without permission of the Court or Probation Department. If he is on supervised probation, then he will have to seek permission to leave the state from the Probation Department (and failing that, the Court). In situations like this, the Court and Probation Department are usually happy to accomodate the request.