Unfortunately, in my view, he might achieve more (from the outcome he is generating) than get fewer rewards by the downvoting of the content he votes for.
And that's because all others voting for the same content will generate a much larger sentiment of "oh crap, what have I done now"...
People will mostly not understand why they are getting downvotes, and if he chooses to keep curating the largest voted posts, people watching it will probably never see things and revolt against whoever is downvoting instead.
This already happened with many other accounts in the past.
Either way, he will always win rewards, that's my point. And there is no real way to "prevent" it, while he has a stake. Obviously, if he authors, then the thing changes.
It is the only thing you can do (and I understand that) to reduce his rewards, but I doubt it will affect him. It would only affect him if we all agreed on getting posts reduced to zero.
But then, lots of people would get impacted, because he would keep choosing randomly voted posts with lots of value. Again, affecting the community in general more than being "affected" himself.
A real problem to think about... indeed.
That's why I love this chain!
Maybe time will curate things... let's see. Not trying to convince you to do anything about this, just being transparent about how I see things.
Cheers