I agree I would tell him. Be open & honest. He will find out eventually & better by u.
Just be honest that you weren't really sure how to tell him but you really want him to hear it from you since it is involving what will be going on in your child's life.
I think you should tell him out of respect. He should know what is going on in his daughters life. If my husband and I were not together, I would want to know what is going on in my daughters life at his house. I feel he will find out anyway so if you tell him sooner rather than later it will be more respectful. My thoughts.