You've already decided to have the child. He had participated in the creation of your baby. There is no wrong or right way to tell him. Now you're in a different situation than the first time when you got pregnant because of his decision to seek divorce.
He is responsible for what had taken place prior to his decision and for what had occurred afterwords.
Just be straight forward with him. you have control of what you do or say and not of his response. No matter what you say or how you do it, he still decides what to feel, say and do.