Most Helpful Guy
There is no right and wrong way for this.
This is why guys are never asking out girls, they always end up being crushed by depression and want it all to end. He sounds like the type of guy who would end it all if you said anything.
What you've basically told us that he's in your friend zone and you can't see him anything more than a friend
What I recommend is that you tell him that "there are other girls out there" and "I'm not the one for you". Or just make up some lie about a friend stating that he has been with some girl. Let your friend know ofcorse.
I recommend you pay someone on the street to say that you're friends with them and have them say that they say him with some other girl. There is no other way other than the manipulative way to help him get over you.
Most Helpful Girl
You have to tell him straight forward.