Who is the current king of Wales?

There is no current King of Wales. The last Welsh King was Owain Glyndwr (I think) and after the English conquered Wales, they had the heir to the throne given the honourary title of Prince of Wales. That's why Prince Charles is called the Prince of Wales.

Once the Queen dies (or abdicates) and Charles takes the throne, he will be the next King of Wales and Prince William will be the Prince of Wales.