TBH, my professional opinion (as an academic lawyer and something of a specialist in defamation), unless you're in a position like Chapman's and you're advised that there's an actionable defamation, I wouldn't consier it necessarily wise to acknowledge that sort of criticism publicly. Rather than a rebuttal, he'd probably have been better off playnig the bigger man and telling his own online following to leave the kid alone, a la Mary Beard.