First, as he was unrelated to your mother by birth or adoption he has no rights to her estate. No one has a right to inherit from either of their parents, any child may be disinherited. This applies both to children born within and outside of marriage. If your father does not execute a properly drafted will we look at California's intestacy statute to determine whether your half sibling has any rights. Although adoption ordinarily severs the parent child relationship for intestate succession purposes, there is an exception under 6451 of the California Probate Code which may apply to this potential competing heir. If your half-brother ever lived with your Dad, as your Dad's son, the adoption will be disregarded and he will have the same rights as he would have had if he was not adopted by another. All that said, your father can still disinherit him. I've attached code section 6451 into my answer so that you can read it yourself.
6451. (a) An adoption severs the relationship of parent and child between an adopted person and a natural parent of the adopted person unless both of the following requirements are satisfied:
(1) The natural parent and the adopted person lived together at any time as parent and child, or the natural parent was married to or cohabiting with the other natural parent at the time the person was conceived and died before the person’s birth.
(2) The adoption was by the spouse of either of the natural parents or after the death of either of the natural parents.
(b) Neither a natural parent nor a relative of a natural parent, except for a wholeblood brother or sister of the adopted person or the issue of that brother or sister, inherits from or through the adopted person on the basis of a parent and child relationship between the adopted person and the natural parent that satisfies the requirements of paragraphs (1) and (2) of subdivision (a), unless the adoption is by the spouse or surviving spouse of that parent.
(c) For the purpose of this section, a prior adoptive parent and child relationship is treated as a natural parent and child relationship.
(Added by Stats. 1993, Ch. 529, Sec. 5. Effective January 1, 1994.)