Does my step-dad have to pay child support even though him and my mom are separated?

I don't think he ever had the obligation to pay for anything for you. He's not your father. Where's your biological father? Is your mom counting two men as being responsible for you? Unless he legally adopted you, ending your biological father's responsibility and establishing you as his. If that's the case, then whether he's seen you or not, he's responsible for you.