Not sure if it is different in England Jill, but you don't need to have the father present if you are married to him.
shorty I agree with the others here, you need to be absolutely sure who the father is before you decide to make any changes (if they are possible). Were you sure at the time you registered him, or is it that the person you chose to put down is not being a good father so you would like to change your mind now?
What makes you think you got it wrong, does he now lok like a different partner of yours?