5yo is a big time for competitiveness. Some of that is just normal, I think. But talking about feelings might help him to get it. Especially if you challenge him intellectually as you also talk feelings.
And I wasn't entirely kidding about the "Does my butt look fat?" kinds of questions. Using that sort of example of an opportunity for honesty vs. kindness can help to drive home the problem to a kid who doesn't see what's wrong with his behavior.
Can DD talk to him about how it makes her feel? That, too, might help. (Or it might be seen by DS as an opportunity to torment her, so use your judgement there...)
Gotta love those teachable moments!
