Thank you for the additional information.
You can also file a police report because what he did fits the definition of a crime in California:
422. (a) Any person who willfully threatens to commit a crime which
will result in death or great bodily injury to another person, with
the specific intent that the statement, made verbally, in writing, or
by means of an electronic communication device, is to be taken as a
threat, even if there is no intent of actually carrying it out,
which, on its face and under the circumstances in which it is made,
is so unequivocal, unconditional, immediate, and specific as to
convey to the person threatened, a gravity of purpose and an
immediate prospect of execution of the threat, and thereby causes
that person reasonably to be in sustained fear for his or her own
safety or for his or her immediate family's safety, shall be punished
by imprisonment in the county jail not to exceed one year, or by
imprisonment in the state prison.
(b) For purposes of this section, "immediate family" means any
spouse, whether by marriage or not, parent, child, any person related
by consanguinity or affinity within the second degree, or any other
person who regularly resides in the household, or who, within the
prior six months, regularly resided in the household.
(c) "Electronic communication device" includes, but is not limited
to, telephones, cellular telephones, computers, video recorders, fax
machines, or pagers. "Electronic communication" has the same meaning
as the term defined in Subsection 12 of Section 2510 of Title 18 of
the United States Code.
The police then may decide to charge the individual. I would suggest if you decide to take that route to print out a copy of the Facebook message to show the police so they can decide how to proceed.