As everyone who follows the 6 principles will know, this is an extraordinary claim. Does he offer extraordinary evidence to support this claim? Simply put, no, he doesn't The only evidence that he has supports only that the photo indeed originated from the Civil War era by a photographer by the name of Professor G.B. Smith. Another thing that may come into play here might be apophenia - the tendency to perceive meaningful connections among unrelated phenomena. The phenomena, in this case, is that they look very similar, the man in the picture and Nicholas Cage. However I think that because our facial recognitions focus is on the similar parts of the facial features, it ignores the parts that are not similar.
In all, I don't think that he is really trying to make anyone believe that this truly is Nicholas Cage, but rather to point out the striking similarity that they do have. This, along with a small story that made it go viral, is an attempt to make a large amount of money off of something that otherwise may be considered nearly worthless.