#人生半途 

Looks like handmade paper for watercolor. Would love to know what he actually uses.

The style for anyone to know is Ink zen wash. Paper, xuan paper (Chinese) or washi (Japanese paper) both of which are highly absorbent and unsized. Silk is also used in some forms of ink painting. Many types of xuan paper and washi do not lend themselves readily to a smooth wash the way watercolor paper does. However, you can experiment with watercolour paper if you like, put water on surface of paper and make sure you got paper pinned down with painter tape, and clip for good measure. Experiment and see.

@@TheMediaMachine thank you so much! And you mentioned that it's 'ink' so can you please tell us what kind of ink it is?

@@TheMediaMachine 감사합니다^^