The image that the artist first created, whether digital or analog, is the “original” for copyright purposes. He can make as many copies as he wants afterward—good luck trying to enforce an oral promise he won’t. What you basically are buying is a recreation of the original. For the future, it would help if he wrote something on the back saying it is a one off duplicate of the original, and he won’t duplicate it again, not so much for legal enforcement purposes (unless you have the foresight to buy from the next Jack Kirby), but it should add to the market value. IMO.