This guy likes me but I don't like him back, how do I get him to leave me alone?

[Balaloop]

I'm 17. There's this guy in my English class that I sit with. We used to do all the group work together because neither of us had any friends in that class and we'd chat a little but it soon became more like a friendship and we'd say hi to each other if we were passing in the corridor. We come from two really different groups of friends though. Him and his friends like to talk about video games and stuff and don't go out on the weekend. I have more of a social life. I have NOTHING in common with him. It became really obvious that he likes me. He drew a picture of me and gave it to me. I made it clear that I wasn't interested and told him that I like someone else. But he said we could still be "good mates" and he has my number and talks to me every time i'm on facebook. He's a nice guy but I need space. I don't see him as a best friend how he apparently sees me but it would be sooo mean to tell him that. He's now wanting to hang out with me out of school and he texted me the other day asking me to go to the mall with him after school and I said I couldn't and he got all offended like "You can just say if you don't want to " I feel so bad but I find him soooo boring to be around and my friends think he's weird. I feel shallow for even saying all this because he's a good guy! I don't want to hurt him but it's driving me crazy. What do I do?