Your boyfriend is likely facing the charge of a violation of Penal Code 422 - commonly known as making a "criminal threat". This crime is a wobbler, meaning it can be charged as a misdemeanor or a felony. If it is charged as a felony, the maximum possible time the statute allows for this offense is a three year sentence (assuming your boyfriend has no prison priors or prior strikes etc..). Also, know that if it is filed as a felony, it is being charged as a "strike" offense, and if convicted of a strike offense that can greatly impact the amount of time your boyfrend could be sentenced to in future felony cases.
If it is filed as a misdemeanor, the maximum time your boyfriend could be sentenced to is a year in County Jail. And it would not be considered a "strike" offense.
HIs result with depend on various factors, such has his criminal record or lack thereof, the strength of the evidence against him, the specific nature of the threats he is accused of making, whether he was experiencing mental illness manifestations when the incident occurred, his attorney's persistence, knowledge and experience, and several other factors.
Unfortunately for your cousin, the Riverside County DA's office is usually very aggresive in prosecuting any offense that could be considered a strike offense. So - what to do? What can you do? Help your boyfriend hire an attorney, or if he has no means to hire a private attorney, then he may qualify for the assistance of the Public Defender's office.