You know, while I understand why you'd want to keep him around, isn't it a bit unfair to do so knowing that he's going to want more from you and probably infer more about your relationship from the time you spend than he should?
While I don't that you have to be fully responsible for other people's feelings and decisions - you do need to be straight up with them in the first place. You like him for what he brings to your life: you get your emotional needs met without doing any of the hard work. But you know there's no way you'd be interested. But flip the situation, if you were in his shoes, wouldn't spending all of this time together make you think that there was more than just friendship given that he knew how you felt? Exactly.
Now consider that you're trying to figure out how to kick him into the Friend-Zone while still getting what you need out of him and well, I'm going to suggest that maybe you stop spending so much time with him and eventually fade him out altogether. You've seen the movies and episodes of The First 48 and Law & Order where these things end badly.
Feelings can't just exist in limbo. They need an outlet. Either they get reciprocated or they get deflated. You have none. He does. I know it's not fair and a bit abrupt but chances are that you can't keep him the way you want without leading him on a little bit.
Of course, if you just tell him there's no way in hell you'd date him and let him decide, then have at it, just realize that might also end badly. But at least you were straight up and honest with him.