The parole board can give him up to a year for the parole violation.
It is also a crime to threaten to kill or seriously injure another person. If he is convicted of criminal threats under Penal Code 422, he could be sentenced to as much as three years in prison, plus an additional year for his prior prison term.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like Avvo, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.