You can not reason with him so don't even try. From what you wrote it sounds like he has a diagnosis of Paranoid Schizophrenia. And with that decrease in Haldol, no matter how small, it can certainly not help - but increase symptoms.
A good response for you would be something like "I understand you feel that way but I am not hearing any voices now but I know you do". Of course, said in a very gentle, non-confrontation way. Bringing him back to reality - right now, is not possible. Do not confront him or say anything to the effect "you are very sick, there are no voices". It will not help but hurt the situation. Be accepting, supportive and loving. Distractions of any kind might help him ground himself(?) Right now is there anyone (family, friend, religious support, etc.) you can speak to and let them know of your present situation with your son? You need support, if nothing else they can provide you.
I more than hear you about calling 911 yourself. No parent wants to be the "bad" guy, no parent wants to feel they "betrayed" their child. An extremely difficult situation. Try calling a local chapter of NAMI who can advise you.
And certainly notify his psychiatrist and therapist if he has one, does your son have a hospital "portal" in which you an even email his treatment team? Is he associated with a mental health clinic?
Call them and report everything going on with him and you fear of calling 911. They should advise you.
In situations exactly like yours - ALL families are in a terrible dilemma. Try to put the "onus" on a professional in calling 911. You are living with him. Try to step back and correctly judge his situation. He is deteriorating?
Are the voices increasing? Is his paranoia increasing? Is he taking his medications? Is he sleeping, eating...is he in a constant state of agitation? Threatening to you or others? As hard as this is, if all what I just wrote is true...an intervention is warranted.
Try to reach out to someone to start action. I don't have to tell you he needs help. His psychiatrist must know that his/her recent medication adjusted for him has had a negative effect on his mental status - not positive.
His doctor must know this! All the best.