just downvote two of his other comments.

No.

Karma is not just about reward/punishment, it's also about information. The karma score of a comment tells me how many people found it worth reading, and how many people found it harmful -- and comments can be sorted for that. Downvoting comments one wouldn't otherwise have just because of who they were written by amounts to injecting noise into that information. Please don't do that.

What's abusive about it? You'd rather he lose more karma from a mistake that he's already learned from?

In general, I'm in favor of people acknowledging past mistakes, rather than pretending that nothing their past selves did was in error. Even when the acknowledgment is of the form "oops, I suppose it was a mistake to voice this opinion in this forum" rather than "my opinion was wrong".

I'd really prefer we didn't have a social norm that changing your mind in response to other people's input is a bad thing, even when sometimes people learn a different lesson than the one we wanted them to. Besides, is it really so much worse for someone to learn the lesson "this sort of thing results in karma loss" vs "that comment didn't contribute to the site and wasn't worth posting"? Are they really such different lessons?