I cannot know for sure if he needs medication but if he is having anger outbursts then he should at least be addressing these symptoms with his doctor or clinician. He should strive to maintain a close connection with the professionals who are treating his condition.
With regard to medication, Zyprexa is expensive. A month’s worth of Zyprexa can cost nearly $400 (as of a few months ago). Few people can afford this expense. Zyprexa is not only expensive but it can cause weight gain, as you mentioned and it has been linked to type II diabetes. While taking Zyprexa may lead to serious physical health issues, many find that it effectively controls the positive symptoms (e.g. delusions, hallucinations) associated with serious mental illnesses.
Your son should talk to his doctor about either getting samples (which are free and many doctors have them to give out) or switching to a medication that does not cause weight gain. Depending on the nature of his symptoms, there may be at least a few other medications he could switch to.
Please know that it may take some time to find a medication that he can afford and that causes the least amount of side effects. Finding the right medication is a trial and error process and patience is required from both from the doctor and the client.
He could also try seeing a therapist who can work with him to teach him how to control his anger outbursts. It’s best that he see both a doctor and a therapist. This comprehensive approach to treating his bipolar disorder is highly recommended.