New cat attacking me and growling

Yesterday I adopted a cat that had to be rehomed due to the previous owners dog that messed with the cat. The dog had chased and scared the cat so much that he had peed himself and refused to leave the one room where the dog was not allowed.

Yesterday we brought him home and he was so sweet like he was when we went to see him for the first time in the previous home. Cuddly, friendly and purring. This only lasted for few hours and after he realized we had another cat in the house his behaviour changed. He sarted hissing and did not want anything to do with us.

I had watched a lot of jackson galaxy videos and they are in separate rooms but can obviously hear each other because my doors are not soundproof. So like in jacksons videos, I gave him space, treats and assosiated myself with positive things. Worked a little and he wanted pets again.

Then today did all of the things again, sat in the middle of the room, using calm voice not looking at him and offering treats. He came up all friendly at first but then I did nothing and he started growling and tried to hit me with his paw. I obviously instinctly went a little back and that pissed him off even more. He growled at me, ears back and left.

Sounds stupid but I am now scared to go in to the eoom where he is due to him being so unpredictable. I know a thing or two about body language and he did not seem at all like he was going to attack me. I've never been scared of cats before and I have owned a few of them.

One note is that he is now 3 and has last seen a cat since he was kitten. Could he just be too traumatised about the incident with the dog and is now afraid of my cat on the other side of the door because of that? Maybe he needs a home where there is no other animals? I don't know what to do? I am scared and so is he.🥲