JB...I'm so sorry.
I would call his doctor and probably the police too and report these threats. Maybe offer to take him to the hospital yourself. These comments should be taken VERY VERY seriously.
Your son is well past 21. I'm sorry to say, that I would GREATLY limit your involvement (with the possible exception of certain things). I WOULD call for help if he threatens suicide and depending on your ability, I WOULD do your best to help out with the cost of any medical needs (physician appointments and medication). Likewise, with addiction treatment.
But, especially if he is being disrespectful to you, I would back away with helping out with most things and I would do your very best to disengage from the drama and trauma associated with his bad choices. Of course, he should know that you love him, but at the same time, he should know that you do not wish to have any part in his bad choices and that you wont have any part in them.
He is over age. It is very sad that he has an illness and an addiction issue. You might open the door to treatment, but he has to make the personal decision to walk past that threshold and do the work needed to enjoy better health.
Likewise, you are responsible for your own good health (physical and mental health) as well. And you will not be able to be happy and whole if you are overly concerned with his bad choices...which you have VERY little and probably NO (nada/zip) control of. Consider going to Families Anonymous, Al Anon or NAMI and get support. Take top notch care of yourself and nourish your relationship with your spouse whole heartedly.
Susan Lee....much of this might help you as well. Please consider checking out one or all of these groups. Also, there is a book called "Boundaries" by Dr. Cloud...consider taking a look.
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Big Hugs to you Just Jacqui.