So after a slight skim and reading a bit, up to page 12, just from this manga he doesn't really seem to have a name really. I nickname him Real Lucky guy in this case.
Seems like to me after reading it, Sinon is acting out a fantasy and basically using a stranger instead of asking Kiriko instead.
Real Nice and simple read actually, not as NTR as i thought after i saw the tag. As a Sinon fan myself this and the other looks nice. (Them Eyes)