It's easy to convince yourself that your kids won't care about their differences, so applaud yourself for dealing with this thing head-on. Still, it may not be something you need to tackle right away. Your son is too young to make any real distinctions between himself and his new brother beyond cosmetic ones ("My skin is brown and his is pink"). Before you give birth, go ahead and tell him about the way the baby will look so he won't be surprised, but keep it simple: "He'll be small and he'll cry, just like you did when you were a baby. But all babies look different, so his skin and hair won't look like yours."
Later, when he starts asking questions, assure him that you, his stepdad, and his half brother all recognize that he comes from a different background, and that you all want to be a part of it and celebrate it. Take advantage of books, museums, plays, and churches to learn about the black experience. Reach out to a mom of color and arrange playdates. You might make a friend, too, who can advise you about navigating the tricky terrain of raising a kid of color in America. But most important is that your son spend time with his birth dad's family.
If you're not close with them, get in touch with the extended relatives. And make sure your family treats him with the dignity and respect they give any other family member.
I really believe that if you embrace both sides of your child's heritage, he might avoid growing up feeling caught between worlds (or not belonging to one), something many biracial kids experience.