been there, done that.
1. tell him you won't discuss personal matters like this at work. set boundaries. if he tries to upset you at work, tell him you'll speak with him another time and hang up. be firm. don't let him manipulate you.
2. as others have said, it's time to hire an attorney. you may want to interview more than one to see who you are comfortable with. do you need a shark or someone who will suggest compromises? I wouldn't give up on a negotiated agreement between two amicable adults just yet. if he's starting to get crazy, you may be able to pull him back. but be prepared -- sane, rational adults can turn into raving lunatics in a divorce.
3. recognize that ds has two parents. I disagree with aahmom about shared parenting -- it works sometimes, not at other times. it may work for you, it may not. but you do need to work out a custody and visitation arrangement that gives both of you time with your son.
4. find a real-life and/or on line support group for people going through a divorce.
{hugs}