Most Helpful Guy
Hm, gotta say I hate how you worded this question lol. like you're subjecting him to keep being the boyfriend without the relationship part and you're aware of it. No offense or anything, sorry if it comes off harsh.
But you just tell him and let him down respectfully and easily. You tell him you genuinely enjoy his company, having his friendship but anything further like a relationship isn't possible at this time. As you feel no attraction towards him and it wouldn't be fair to you or him to just start something just to quell his feelings. As feelings should be reciprocated and following both people liking each other thus a relationship can form.
Let him know you weren't trying to lead him on by hanging out, but it was fun and you'd like to continue hanging out as friends. Now be aware...
If he hangs in there as your friend and you're aware of his feelings. You MUST keep it friendly, not any misleading physical contact or flirting of any kind. as it will always confuse him as to your actuons and make him think there's more to things. Goodly like with this:).
Most Helpful Girl
Don't tell him you're friendzoning him. Just act like friends.