Boy troubles(I really need help

Ok hey people pleaze i really need help with this problem!!!!!!!!!!! ok this kid keith asked me out today at school. he's cool and all but he's like my brother. i asked him if i could answer him monday and he said sure. now the only reason i asked was so you guys can help me! i don't want to go out with him because i'm still getting over me and my ex david breakin up plus i just don't want a bf! Keith has been waiting for the day that david and i broke up and now yea. what do i say to him without hurting him?

thank you so much! i just didn't really know what to do but with your help now i know how not to hurt a great kid thanks agian