Fanfic writers always write leather, but I maybe it's rather a rayon(?) kind of firm yet supple cottonwool weave? Leather would be hell to wash, stiffer and hot.
I've spent hours thinking about his clothing already, squinting at the pictures and guessing from the flow and way to rip, what it could be.
How far is the Saiyuki universe anyway, in terms of development? Do they have all sorts of synthetic fibres already?

I think his blackie is enough to cause a fetish. He makes it look so damn good (or the other way around, however you want to see it) :D

Of course this is all bull and we know the truth.. it's paint on latex, and Goku is forced to paint it on him every morning after his shower =)