Some courts will presume that he is the father of the child because the child was born during the marriage.  However, since there is a biological dad in the picture, he does not have to be bound to be legally responsible for this child.  Instead, he needs to file for a divorce combined with  paternity suit so that he can have the new boyfriend officially declared dad.  If he does not do this when he files for divorce, then he could end up being legally responsible for the child until the child reaches the age of majority.  (Courts will argue that because he had an opportunity to contest paternity and failed to do so that he agreed to be dad.... this is mainly because the courts do not like to declare and undeclare dads in such a way that harms the child.)