I think my friendship may be becoming something more?

I have a really good guy friend that i have known for a while now. He is one in a large group of friend at school (comprising of both boys and girls) but i am worried that he is starting to crush on me. Am i being paranoid? He has started to ALWAYS sit next to me and put his arm around me. Its not that i don't like him its just that id prefer him as a friend rather than boyfriend. I cant tell him this because i don't know if he does like me, and if he doesn't then i have effectively given away my friendship and made it look as if i do like him (which i don't). What do i do to keep a friendship?