What is the kindest way of telling an over-emotional ex-boyfriend that you are having another man's baby?

Ouch! Save him from a person like yourself and break up with him. Tell him you are guilty of cheating and he deserves better. There is no kind way to tell him and you will hurt his feelings. But do it soon before word gets around to him. If he still wants you after you've lied to him and betrayed him, then he either really loves you or is desperate. Good luck.