mm, maybe because he's still unsure of their relationship? considering his words in the recent chapter. there's also an age difference, as well. even though they're pretty lovey-dovey with each other, i don't think he sees yuan as a close friend/lover.. >< hard to explain. but it's just a hunch
Isn't blondie who calles uke-kun "Geonwoo-ssi" this and "Geonwoo-ssi" that? I think at first it's because he was his customer from the sex-shop (so he had to address him formally and respectfully) and then once they got closer, it naturally stayed as a seductive nickname. Since blondie's so much more experienced than him, it evens the odds a little.
From what I understood from Korean dramas "ssi" means something like "mister" and it is used to address people formally (not between friends or lovers) so calling someone you're going out with and kissing and having sex using "ssi" is weird