[FairyPoppins]

If you don't tell him, then he won't have a chance to be in this child's life.

That might be fine for the first few years, but what about when the child goes to nursery and realises that they don't have a daddy like most of the other children?

Even if you substitute him with another male, when the child gets older they may want to track down their real father. How awkward is that going to be, when a teenager turns up on the door of this guy and says, "I believe you're my father"?

I think he deserves to know now. Not only will it save him from being shocked in years to come, but it will give this child the opportunity to have their father around. This will help you too, as bringing up a baby isn't cheap and he would be required to help you financially. After all, he helped make him/her.