But the sort answer is that you only need to notify the school he's currently attending that he will not be returning.
California is what we call a private school state; that is, homeschoolers are considered the equivalent of private schools. You will need to file a Private School Affidavit between October 1 and 15, which is a simple on-line form where you describe your private school. You can find a comprehensive guide to homeschooling at California Homeschool Network.
I highly recommend membership in the Home School Legal Defense Association (HSLDA). Although it is legal to homeschool in California, homeschoolers still continue to face challenges from public school officials, and, in some cases, Child Protective Services. HSLDA membership is a small amount each year, and it's worth every penny.
Finally, you might find some helpful resources at NATHHAN..