Who tells the ex?

My ex and I have 2 daughters. We separated 2 years ago, and I've tried to be as friendly as possible since the split. He is very bitter and refuses to communicate with me most of the time. I'm expecting a son with my fiance and I know the news is going to upset him. I can't decide if I should tell him, or if I should just let him find out from the kids after we tell them. What would you do?

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