Maybe theThe reason Brock is odd around Walt is that he's only seen him twice and doesn't know him at all. I remember that when the first time Brock has seen Jesse, he was very quiet around him too. But Brock definitely knew something about poison, when Walt tries to make small talk, Brock doesn't seem to be too enthusiastic about interacting with him. But he instantly got along with Jesse. Brock is only a kid, and he has no apparent reason to not like Walt unless he knows something, right?