Given the character and the type of clothing, It's most likely Broadcloth, like the one in the picture below, and at the end of this answer: In the picture; however, in my opinion, it looks like velveteen. The picture makes the shirt look like it has that uneven surface, like a more "fuzzy" fabric. It's a kind of thinner, more flowy, but still soft, fabric. It's like if you crossbred satin and velvet. Here is a picture of just the fabric: It would most likely be satin over silk, but I don't think it's either of the two.
It's a black silk shirt. You do realise silk comes in different weaves, spins and polishes? There are only two fabrics commonly used in shirts that would give that lustre and they are silk and polyester satin. Now would that character have been wearing a CHEAP, SYNTHETIC shirt or would he have been wearing an expensive luxury piece? I've got a pair of black silk pyjamas and they certainly look like the same fabric! It IS a really comfortable fabric to wear, but it is expensive. My pyjamas were over $200 and they're not even a brand name item.
soft
