Welcome!
When I first read your post I realized that I'm probably in a similar situation right now, but from
his perspective! The situation is a little different (as this guy & I have been friends for almost 3 years & we spend every day together) but you've just got to be honest w/ him. Sometimes when I feel like my friend is sending me signals (like "I want to date you seriously" signals) I will usually approach him about it & tell him that if we want to remain friends we should take it back a few steps. I would guess that your friend isn't as intuitive or maybe he just doesn't want to be but you've got to be honest! And if you want to remain good friends I would drop the "I just want to be friends" line...it just sends the wrong message! Don't you just wish that words would always mean what they are supposed to mean?
Good luck!