Congratulations on your pregnancy op 
No advice just
that a grown man, a husband of 19 yrs, and a father of three could behave in this despicable manner. Really sorry you are going through this.
The most
for me is telling your nineteen year old that he will be leaving as soon as the baby is born. What example does he think he is setting? 
He should not even be discussing this with the DC. Is he talking to the DC in order to get the whole family on side against your decision? If so, again, pretty despicable.
I agree that you would be better off taking control of the situation and asking him to leave now if that is his ultimate decision. I hope pure anger and outrage will keep you strong during your conversation with him. (Tell him that although you understand it's a shock, he is behaving like a feckless teen boy.) And have an answer and course of action planned ahead for any of his likely responses. Keep strong! 
Sorry op, but is he normally such an utter prick? Tell him the mature thing to do would be discussing this with you privately, supporting you, while you discuss all the ramifications for everyone. Anything else, he can sod off. Good luck.