What do I do about this younger guy?

He's a rising sophomore in high school, I'm a rising senior. He had a crush on me for a lot of last year (I know for sure because he told my friend that he did and she told me). I'm not interested in him at all, but I'm having trouble shooting him down because he's never explicitly asked me out. He just asks if I want to hang out (like, just the two of us) which is weird because we're not actually friends, just acquaintances who know each other through our school theater group. I've never said yes, but the other day he IMed me and we were talking about Frisbee or something and he said he had to go and then he said "hang out soon?" and I didn't know what else to say (I couldn't just say "no") so I was like "yeah" (stupid, I know, forgive me, I panicked and I knew that taking too long to answer would be as bad as saying no). So tonight he emailed me and asked if I'm free this weekend, and I am, but should I just pretend I'm not? Keep saying I'm busy until he stops asking? It's possible he'll never have the balls to ask me out for real because of the age difference, and I think he's just doing this because he wants to spend time with me even though he knows he can't date me, but I really don't want to because being around him one-on-one is supremely awkward. Help!!

Oh, and just an aside, I'm actually predominately interested in girls, but I'd prefer not to go there. For one thing I'm only out to my closest friends, but more important, that's not the reason I don't like him. I have liked guys before (rarely, but it's happened) but he's just not my type and too young for me, so I don't want to use the gay thing as an excuse.