In 1996 I came up with the idea of a game that involved catching mystical creatures in some sort of container and using them to battle each other to become stronger. There was a series of competitive places that you would take said creatures and, when you defeated the leader of the facility, you would get a congratulatory medal. When you collected enough of these medals you could enter a tournament to prove that you were the strongest out of all those around you.
Two years later I hear of Pokemon which has a similar structure to it. While I suppose it is possible that, somehow, someone from Nintendo over heard me talking about this game to my friends I highly doubt it. It was just a strange coincidence. I believe it would be the same in this situation and, even if it wasn't a coincidence and he really did steal the design from a 13 year old kid, you'd never be able to prove it with the evidence he has. "Well I have drawings that proves I designed him". Maybe so but there's no way to prove when you drew them. "I entered these drawing in a Doctor Who contest". That may be able to be proven if they would be bothered to look into it but even if it was determined, there would be no proof on whether the person who is credited on designing the character actually went to said contest or even saw the design.
The fact remains there is no viable evidence that he actually designed the character so he should, just like me, just live with the fact that one of their ideas is making millions of people happy and not care about getting a reward. After all, I'm pretty sure the company would just rather agree to never use the character again over paying some random guy off just to use one character in a series with dozens of enemies.