maybe he's frightened that it will change the friendship that you already have. Make sure you don't pressure him into thinking that you have to get together. I'd just sit down with him and explain how you feel. Hopefully he'll open up and explain what he's feeling.
The only way to get through issues like this is to communicate.
Maybe he just doesn't fancy you? Sometimes you can be really great mates with a good looking person of the opposite sex, but still not feel 'that way' about them. Don't push it, just value your friendship. He'll make a move if he wants to.
I agree that he may just not fancy you. It's perfectly possible to really like someone's company without finding them attractive. Before you do anything you need to bear in mind that revealing your feelings will change the nature of your friendship. If he's a grown man, he will probably just be flattered and will get over any awkwardness if you get back to acting normally with him straight away but, if he is a teenager or young adult you should be cautious, he may react quite immaturely. A real friend of the opposite sex is a great thing to have.
Maybe he doesn't feel ready for a relationship. You have to take to him and see what his problem is (in a nice way of course!) and then you'll know where he stands.
If he wants to wait for a while take it as a compliment and then decide what to do.