Well....he looks quite like him. of the two of them, I'd say William is the more likely to not be Charles'. Harry WAY takes after the Windsors.
William has the Windsor hairline and, while he looks much like his mother and uncle (Charles Spencer), there is also a strong resemblance to Peter Phillips (the son of Princess Anne)
I don't think Harry looks that much like a Windsor. Of them all he looks most like Prince Phillip, and I feel his seed was sown far and wide.