Why is he doing it? I know you don’t have an exact answer, but there’s got to be something behind his behavior. Especially when he’s ignoring negative consequences.
Is he expressing something, some kind of legitimate anger or frustration? Somehow enjoying negative attention because he’s not getting enough positive attention? Those would be my first (uneducated) guesses. He’s meeting some kind of need with this behavior.
And the “enough” positive attention isn’t meant as a criticism of parenting; it’s possible that one child just has a really high threshold for need compared to the other.
It’s so easy, when you have two kids, to look at the one whose behavior you like and call that “normal” and see the other child as “aberrant”, when really they’re both acting out two different versions of “normal”. Again - that’s not meant as a criticism, it’s just something that I’ve noticed in myself, something that’s so apparent when you have twins. What’s the overall dynamic like?
And you know, that age, 2 yrs 9 months, is a real bitch. I screamed myself hoarse more than once, in a vain attempt to get mine to behave themselves. Any problems they had beforehand are magnified 10x for a while there. It does pass. Comes back at around 3 yrs 6 months, in my experience. Louise Bates Ames has a lot to say about those 1/2 year periods of “disequilibrium”.
My daughter bit my son on several occasions, especially at that age. She was seriously angry with him, he’s a notorious toy-snatcher, has been since he could crawl. She hasn’t bitten him in, gee, it’s been months. I wouldn’t say that any one thing “fixed” the problem, everything between them isn’t solved. They often play for hours, though; there is hope in the passage of time.
It might help to get a “big picture” sense of this, because I can imagine in the moment everyone’s just on edge; nothing’s worse than having one of your children (or grandchildren) hurt the other. What’s the pattern?