Most of the literature on polythelia is quite old. Extra nipples are more common in women than men, but even then are only 'diagnosed' in about 2% of the female population. The extra nipples can vary in appearance from a patch of hair to full blown polymastia. Early studies suggest a dominant inheritance pattern, and Alexander Graham Bell was successful in raising sheep with multiple extra nipples through selective breeding.
So is it a third nipple? It seems to be on the milk-line which runs from the armpit, through the real nipple and down to the groin, which supports his case, but I think we may need to check his parents.