Penal Code 422, criminal threats, is a "wobbler" that can be charged as a misdemeanor or a felony. As a felony, is carries up to three years in state prison, and a misdemeanor has a maximum of a year. A felony 422 conviction is also a "strike" under the Three Strikes law, which can increase the punishment for any future conviction.
However, since he is seventeen, his case would most likely be handled in juvenile court. The juvenile courts focus on rehabilitation, not punishment, and he would probably receive counseling and supervision. There is a possibility he could be sent to a group home.
To convict someone of Penal Code 422, the prosecution must show the defendant:
1. Made a statement, intended as a threat;
2. To kill or seriously injure the victim or a member of their immediate family;
3. Causing the victim to fear for his or her safety.
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 this site, 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.
Mr. Marshall is spot on. The only thing I would add is that, as a former prosecutor, these are often very tough cases to prove at trial as they are word crimes which are always subject to interpretations.