Baby's last name

Me and my daughters father are not married , but we are together & do plan on getting married someday , his son that he has currently has his mothers last name ( son my boyfriend had from a previous relationship) my boyfriend really wants are daughter Marlee to have his last name but I'm not 100% sure , I know if we get married it can be changed to his of hers is mine until then but I also know that he will always be there regardless & will always be her father no matter what ... Opinions ?