@MonsteraDeliciosa and sorry for the long post but I just wanted to give you some encouragement. I think sometimes dogs arrive in our lives, in not always the best of circumstances, for a reason. In my case it was a 5 year old Black Labrador female. At the time I had 2 male Border Collies, only 3 months age difference and best of mates. I also had a very young female BC and a few weeks earlier I'd had to have my eldest female dog, Lab x BC put to sleep. The two girls had also been best of friends and my young female Collie was lost without her.
Unexpectedly, I received a call from a friend who knew a couple with the Black Lab, they were moving house with their two male retrievers and were threatening to have the Lab put to sleep the following day! When I asked why apparently it was because she caused fights with their male dogs?
I found this highly improbable and just an excuse and I was proved right as she never once in the almost ten years I had her caused any issues with other dogs, including mine.
My initial plan was to see how things went and find her a good home if she didn't gel. However, my young Collie fell in love with her virtually overnight so that was the end of the plan!
The first problem I discovered was she wasn't fully house trained. She poo-ed overnight. I played with her meal times until I found what worked and that solved that problem.
Next, I discovered she had absolutely zero recognition of her own name. I've never come across that before and it did tell me she can't have had much in the way of meaningful attention. So I renamed her and within a couple of days she was responding to her new name.
There were some obedience issues too, she was pully on a lead and her recall was hitty missy. She also jumped up. I worked hard on those and resolved them all.
Like you, in the early days, I questioned the sanity of my decision but I stuck with it for the sake of my young Collie and I'm so pleased I did. She turned into the most wonderful dog. On the one hand, she had a superb temperament, I could take her anywhere. On the other hand, she was a fantastic guard dog, and extremely loyal to me. More than once she saw off someone who was up to no good with a deep I'm- not- messing growl.
I always felt safe with her.
Hope was 5 years old when I took her in. They say you can't teach an old dog new tricks. You can,: it just takes longer. Your pup is only 11 months old. I'd be tempted to treat him as though he's just a couple of months old. Start from scratch, assume he's had no meaningful attention or kind, consistent training. Another PP said she feared he may have been abused. I had the same thought and I wouldn't rule it out. But you can give him a new fresh start with a loving, kind home. I'd also rename him in keeping with his new life as his old name may have negative connotations. It's easy to do. Just couple his old name with his new name - Rex-Sam, Rex-Sam - for a few days and then drop his old name.
I hope this is helpful. Please continue to update us with your progress x