A "wobbler" is an offence that can be charged as either a misdemeanor or a felony. HS 11359 (which is the sale or transportation of marijuana; 11358 is possession for sale) is not a wobbler - it is a straight felony. Now the judge might have been wrong in that you CAN move for a post conviction dismissal (there is no such thing as expungement in California) without having it reduced to a misdemeanor. The question is - why? As a minor, he can petition to have his record sealed, which is eminently better. Was be being assisted by an attorney? You should go back to that person and get his record sealed.
Mr. Feasel is a former Deputy DA in the SF Bay Area with over 10 years of criminal law experience. Nothing stated on this site shall in anyway be construed as legal advice, or as creating any attorney/client relationship. If you would like to hire Mr. Feasel to further investigate your situation, feel free to contact him thru this site.
Procedurally you have it all wrong. Hire an a lawyer and be done with it. What you are wanted to do is SEAL the juvenile record. You already made too many mistakes. This process should be of minimal cost.
The above information does not establish an attorney client relationship nor is it meant to provide legal advice.
It sounds like you are trying to do a lawyer's job without a lawyer and messing things up. All you have to do is petition the court to seal his juvenile record, now that he is 18. Hire a lawyer to do this. Good luck to you.