A. Your question got cut off but I think you were trying to say that they do not get along. If I misinterpreted your question, please re-post so we can answer again, addressing the issue.
Changes in household dynamics can be stressful, but this has been on going for quite some time now.
This sounds like it could be stemmed from a health issue or a behavioral issue. I would always start by ruling out a medical explanation (ex. Pain) by getting a check up. If his health seems normal then I would move to treating a behavioral issue. It could be territorial/aggression causing the issue. I would try to get a cat behaviorist called in to evaluate the cat. Aggression can be influenced from MANY things; changes in the household, addition of new people or pets, hormones, stress, fear etc. They will be able to work with you to try and find the route of the problem and a solution.
If you can find it online or on TV "My Cat From Hell" or " Psyhco Kitty" has a few episodes with quite aggressive cats that do not get along. They offer some really great tips to work on these issues and will help give you some background knowledge as to why this happens and how to work with it.
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