Mean how? There's quite a continuum between, say, making loud noises to see them jump, and torturing them.
In any case, though, although you didn't offer much detail, it sounds like a turbulent time in his life - you mention he will be moving in with you, and that he has a new, perhaps somewhat trying half-sibling. Children tend to really benefit from some good counselling when they go through life changes like that. It sounds like overall he is definitely showing some aggression, and maybe it's a reaction.
You don't mention how his relationship is with you overall, but I agree that not hurting cats 'because' they belong to your step-mom is a pretty loaded way to deal with the situation, and may backfire.