Your step-son is lucky to have you.
When children ask questions (and they always do..) sometimes it's hard to know how to answer them. My rule of thumb is to answer honestly, yet not offer any more information beyond what they're asking. So when he asks about his biological father, tell him the truth but not necessarily every detail.
The most important part of this issue is to understand why your step-son is questioning now. The answer lies with your son's self-perception, as he begins to figure out his own identity and personality. He's questioning because he wants to know more about himself.
So for instance, "Who is my father?" Answer, "Your mom said he was a nice guy but actually, I've never met him.
"Why isn't my father around?" Answer, "Sometimes it takes us time to find the right person to marry. (If you were married previously, you can say the next sentence.) You know already, that I was married before and although my wife was a good person, we just weren't the right fit. When I met your mom, we knew she and I would be right for each other.
So it's the same with your biological dad. He is a good person but he and your mom knew they shouldn't be married because they weren't the best fit."
"Sometimes, when people aren't in a relationship anymore, it's hard for them to get together and see each other. It makes them feel badly. So my guess is, your biological dad isn't around because it's difficult for him. Nonetheless, all of us make mistakes, including good people. Your biological dad made a mistake by not getting to know you."
"What I do know for sure, is how much I love you and how happy I am that we are a family! And I will always be your dad and in your life."