Important to note that his Instagram ripoffs haven't been tested in court. He has prevailed in previous court cases with art pieces that had a stronger "transformative" argument, which is a key aspect of fair use. This time the infringement seems more clear-cut. He's not transforming anything, but merely adding his own caption to an existing work. The original work is unaltered.

I was talking about this with a friend, and she said if that was sufficiently transformative for fair use to apply, then one could simply write notes in the margin of a book, call it art, and claim ownership of the book's contents.