I suppose it’s always going to be subjective unless he goes on Maury one day.
I believe you can be certain that the royal family has had DNA tests done, given the rumors, since for years until George and Charlotte were born, Harry was fourth in line to the throne. If something happened to Charles and William, there is every likelihood that Harry would be appointed Regent until the George was of age.
If Harry were not theirs, the Windsors would not be treating him half as well—they stripped Diana of her title at the divorce; if he were a bastard, they would not be letting him have a big wedding with all the trappings.
Several years ago Charles, to cut expenses, sidelined his own brothers Andrew and Edward, and his nieces and nephews by them including Princesses Beatrice and Eugenie, cutting back all of their allotments from the privy purse and not including them on the balcony for many occasions. He is so emotional and loony, there is no way he would be able to fake it if Harry weren’t his.
Think about it.