A medically supervised intervention is basically "medical arrest". Check with your local law enforcement agency. You can request an officer to be present to assist you, due to the propensity (likelihood) of a serious threat. You could also consider placing him under citizen's arrest, whereupon the officer can escort your son to be evaluated. For his OWN safety.
Option #2. Get an injunction (restraining order) against him. If he violates the restriction, he can and will be arrested. Once he is in custody, medical and/or psychiatric treatment can be considered and administered as needed.