Sending you a hug.
You aren't equipped to handle his suicide threats nor should you try. Call 911 or a suicide prevention hotline and tell them what you've posted here. Him quitting his job is a big red flag. He needs help from professionals.
You can then know that you truly have done what you could. Unless you physically kill him yourself, you are NOT "the cause" of whatever happens with him.
You're 23. You're far too young to give up on your life because of his threats.