The problem with using that drawing from the art contest to prove whether or not it's his work is that while it might not be recognizable as any famous artist's work, it could be any artist's work that was posted somewhere on the internet. Also, the fact that the style of that drawing is so completely different from the style of the paintings he has posted, as well as the style of that Image comic book cover, combined with the fact that he won't show us any of the sketches he is supposedly working on just makes everything seem too suspicious.
This is one of the problems with showing art on the internet. Unless you've got a watermark on your art, which doesn't always work, it can be difficult to prove who really owns the work. An art site called www.conceptart.org has had problems with this in the past. People have actually stolen art posted on that site to use in their portfolios to get jobs. What I'd like to know, is how did they expect to perform at work when they got hired with stuff they aren't capable of producing on their own?
Maybe Zodd can prove me wrong; I don't know.