Let your boyfriend be that father figure in his life especially if dad isn't around. If dad is around still your boyfriend be like a dad to him. Let's say you guys get married then he would be step dad and they would already have that relationship. See it as a plus that he wants to be. Most guys don't want the responsibility of another mans child
I would still be in contact with the real daddy. But it depends on how long you have been with your boyfriend. I would wait until you are married. But when you are married, he pretty much is the daddy. Good Luck!