I think that he has to to wear some kind of armor over his chest so he can strap the shoulder armour somwhere, and as it seems flexibel i think it's leader. Another theory is that it's made of some kind of hightech stuff that we don't know. I often though about it because whenever he gets hurt his clothes always seem to repair themselve (in the movie bloodlust Meyer's clothes recover as fast as it's owner.) or have you ever read somewhere that D is sewing his clothes after he got cut, impaled or burned.
I do leather work and I don't think I can get it as skin tight as what D wears... Lol So I like to hear on the most likeliness on what D's outfit consist of.
Well I also work a lot with leader because we have a carriage driving scool. I can tell that the harness' adapt temselves to the horses up to a certain point but I also have my doubts that they could fit that well (unless they were so thin that they couldn't have any use as an armor.)
As for the rest of the clothing it seems to be some normal material. The boots surely are leather and the scarf is knited (perhaps a gift from his mom
). Well as for his underwear... I don't really want to imagin D in speedo so I will just drop that subject