Bringing a new cat in can seriously upset the balance, I assume he is neutered?
I am suffering too but think its more the new cat that is too blame and he will have been here 2 months next week.
How did you do the introductions and for how long?
Some times introductions can take a very long time and know it took a very long time with my original two birmans. A level of toleration was gained which in my twp big moggies heads.....well one of them lol, was actually much more and when Napoleon went to the Bridge in May, Misa moggy missed him loads.
There some occasions when the new cat just cant be integrated but I am hoping that is not the case here and also with you.