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Hi. Sorry you are having this problem. It's very difficult when everyone isn't getting along. Have you heard of a product called Feliway?
It is a very safe pheromone product that can help with a variety of behavior issues, including anxiety and aggression. It comes in a variety of forms including plug-ins for wall stickers and sprays. In this case, a collar for the kitty who is the aggressor might help "take the edge off" his aggressive behavior.
I would also recommend looking into one of the all-natural calming treats or supplements which can also limit aggressive or other unwelcome behaviors. These can be found in pet stores or online.
Good luck with it all. Let me know if you have any other questions about this. Take care and Happy Holidays to you too!