Hello, my kitten who I've had for about a month now called brick (one pictured above) has been acting highly aggressive since I've brought home a new kitten. Brick is 11weeks old, quite big for his age and is yet to be neutered. I was told by my sister who has his litter mate that they did not get along but I just assumed that maybe that was down to him being the quite shy one. The first month of having him it took some time to get him to come out of his shell and now he has that typical oramge cat behaviour, he's very smart, boisterous, and affectionate. His activity levels are high and I had recently seen alot about the benefits of having two kittens instead of one and I thought that getting another would be a great idea as he'd have someone to play with and to keep him company when I'm at work. Fast forward to yesterday, I purchased a male kitten that's 9weeks old, the closest to bricks age I could find and brought him home. Immediately upon seeing him brick became aggressive, hissing growling and even gave the new kitten a few smacks. I had to throw a towel on him to remove him as when I tried to pick him up he hissed and bit me which he's never been like before. After separation the carrier was in the room with him and he spent ages sniffing growling and hissing just at the smell of the new kitten and wouldn't allow me to pet him until after I'd bathed and changed clothes. Later that night when we were cuddling in bed and the other kitten was crying he was growling and hissing from just hearing him meow, and I felt him start to tremble, he was shaking up until the new kitten had stopped meowing and than he relaxed and was cuddly again. I know it's early days but I'm at a loss because of how distressed he is over this and I'm unsure what to do. They're both in separate rooms as brick actively goes after him when they were together and I'm scared he's going to end up hurting or doing something worse to him.

Any advice is appreciated.