I'm not ready for a step-dad?

Having a stepfather is NOT that bad - and I completely understand why you're so nervous about the whole 'boyfriend' situation. Your mom's boyfriend could really have an important spot in your life, and maybe he try to fill the spot where you didn't a real father. Having a dad in general is a great thing, too. Just keep your head up, love, you'll get along just fine. He sounds like a nice man, and I'm sure things will turn out great for you and your family!