I couldn't care less about diana or any of the 'royals' for that matter, but I don't think he's Hewitts. Have you seen a pic of Dianas vulture brother? he looks just like Harry, so I'd say Harry just gets his colourings from the spencer side.
None of this actually matters; Prince Charles obviously accepts Harry as his and adores him. Diana is not here to defend her reputation. Anything in the press casting aspersions on his parentage must be extremely hurtful to the entire family. The only other person in this scenario is Hewitt, and he really is utterly irrelevant.