This, of course happened a lot in the past, before dna and there's many a father who doesn't know he's rearing another's child. All depends on you and your conscience. If you keep it quiet you may be all right, but if the child ever needs serious medical attention then the truth may come out, although I do believe that many doctors will keep it quiet unless absolutely necessary. You will be living on tenterhooks all the time. The ball's in your court and only you know what you can do.