What will Prince Harry be known as when Prince William in the prince of wales?

That depends if they remain in the US or return to the UK. If they choose to return to the UK, his status could be returned to him and his wife, or if they remain in the US, his Royal status as it is now will remain the same. As of now their HRH status has been removed. They have stated that they haven't quit permanently, but have stepped aside from the Royal family.