Welcome to our forum.
"Later on, after he has moved on, I can have some wonderful memories and fantasies of what could have been."
Gee; that actually doesn't sound like such a happy ending ... wouldn't you always be haunted wondering what could have been?
Sometimes I think it's better to take a risk, as true love is always a risk. As they say, better to have loved and lost, than not to have loved at all? I think I would approach S, even at the risk of a heartbreak. Perhaps you could approach him with it in small bites? Such as, starting to describe the depth of your feelings for him, while not doing so all at once?
Of course, you do want to be somewhat straight upfront with him as that is more respectful, trusting, and well, honest. You haven't known S for a long time so maybe your trust levels are more shaky than they seem to be? NRE (New Relationship Energy) can really put rose-colored lenses on things.
I don't suppose it's an urgent matter, so you can take some time to reflect, and decide what you do or don't want to risk telling S. But that's my first thought on the matter, is to err in the direction of telling him (if err you must). You should receive my counsel with a grain of salt, as I am a hopeless romantic.
It seems to me like you have your terminology learned well so far ... I hope you can get the things you envision and desire.
Let us know if we can be of any help!