The behavior you described is worrying, especially from a safety perspective, not only for him but also for others in the household. If you have not done so already, it is important to have a safety plan in place for the other children, should your son become a threat. They need to have a safe place to go, such as another room with a locked door. Even if you are present, it would be reasonable to have them go to another room for their safety and allow you to focus on your son.
This is important: If your son threatens to harm himself or anyone else with any type of weapon, immediately call 911 for support and assistance.
You didn't mention whether your son is seeing a therapist or counselor, taking any medications, or has an individual education program (IEP) at school. Given the behavior you described, these would be reasonable approaches. If he is seeing mental health professionals, reach out to them and address the behavior. If he is not, then we strongly suggest seeking professional intervention and treatment.