How can i say no to this guy?

If you like him, go for it. If you don't like him like that, you will NEED to learn to say no. Just kindly let him know that you would like to hang out or get lunch sometime, but you really don't see him as anything more than a friend. Don't try to make him feel bad...just be gentle. Everyone is bound to be rejected...it happens. Tell him he's a great friend, and you don't want to change that. If he's really that good of a guy, he'll understand.