Your son is delusional most likely due to the fact that he is using methamphetamine. Obviously, he needs to be in treatment and should stop using drugs, but it sounds as if there is little chance of that happening without outside intervention.
Nothing that you could write in a text message would likely alter his behavior. If someone is delusional, they are psychotic. Psychosis means having a break with reality. An individual who is psychotic lacks the ability to be logical. Attempts at logic will almost certainly fail because of their inability to think clearly. Psychosis also makes it difficult to control one’s behavior.
Your son might need to be committed to a hospital. His threats against you may make him eligible for an involuntary commitment but that might be a difficult case to make given your distance.
Are there friends or family who live near your son who can assist in this situation? If so, contact them. Another idea is to call his local mental health crisis center and ask if they would go to his home and conduct an evaluation. You may also want to consider calling his local police department and request that they do a safety check. If the police believe that he is unstable or in danger, they can escort him to a psychiatric hospital where he can be evaluated and protected.
This is a difficult situation. There is no easy or convenient solution. Your son clearly needs help. My recommendation would be the call the local authorities or the mental health crisis team and see how they can help. I am afraid there may be little else that you can do.
If you have additional questions, please write again. Please take care.