It doesn’t really matter whether Harry is, or is not, the biological son of Charles, because Charles has completely accepted Harry as his son, and has been there as his father every step of the way. For what it’s worth, I think he is the son of Charles, but it simply doesn’t matter what my opinion, or anyone else’s opinion on the topic is, because the royal family and Charles accept Harry as the legitimate son of Charles. He is in the line of succession, fifth I think at the moment, as are his two children, sixth and seventh. Hopefully none of them will ever accede to the throne, and as the children of William grow up and become parents, Harry and his children will become irrelevant as they won’t ever accede to the throne and they won’t be doing any royal duties.
I would suspect that DNA evidence has been sought, just to put their minds at rest, but maybe not, because Harry was never destined to be King, and adopted children are loved just as much as natural born children, though as I said, I believe Harry to be the biological son of Charles.