2nd baby worries

My fiancé and I are pregnant! I have a son, Ryker, from a previous relationship who will be 4 in September. Marcus, my fiancé, has been more than I could ask for with stepping up and being Rykers father. Of course I am still worried about how things will be once the new baby is here, since this is technically Marcus' first born. I worry about their relationship, especially if the new baby is another little boy. Any thoughts or experience? I need to hear that it will be okay!  thanks ladies.