Oh, Brittany... I am so, so sorry to hear that.
I think it's always a gamble when taking in an adult dog w/an unknown (or abuse) history.
Let me tell you about Luka... when I first brought him home he experienced a ton of changes, all at once. He became an "inside dog", he didn't have to worry about food, toys, treats... he had his very own crate and blanket, he received vet care for the first time in his life - you know, ALL the basics EVERY dog deserves right from the start.
For the first six weeks he was the perfect houseguest - he got along well w/the other dogs, after a few accidents he learned to go outside to potty, he shared common beds and toys without a problem, and he slowly got used to our daily routine.
Because of all his physical limitations we went to hydro therapy a few times, and Luka seemed to enjoy it immensely. He was also very good around my vet and any "strangers' he encountered, like the physical therapist, and the neuro doctor. Just perfect in every way.
Which was remarkable in itself, given what he went through before he came to me... and everyone who met him remarked on that, how very good-natured he seemed to be, despite his previous experience w/people.
Once the "honeymoon period" was over however... things started to happen. Small stuff at first - he started mouthing me at feeding time or when he demanded attention (like wanting to be petted), and we worked on that for a while until he realized that his behavior wasn't accceptable.
He also started to get very snarky w/Sepp and Faust... he grabbed Faust by the face and neck twice, and he nailed Sepp in the shoulder and neck... blood everywhere. None of the attacks were provoked by the boys, because I was in the yard all three times when it happened. Faust just yelped and came running to me, but Sepp managed to get Luka on his back and was ready to "finish" the fight. Luckily I was able to call him off, but it scared the crap out of me.
After those incidents I started to crate and rotate immediately.
Then Luka started to get very defensive when crated... he would try to "attack" anyone who happened to walk past his crate (dogs AND people) when he had a bone/treat/toy. I moved his crate, so foot traffic was minimal, and that pretty much solved the problem.
A few weeks later Luka was out in the yard, and grabbed the boy who was mowing my lawn by the thigh. He knew this boy very well, had played w/him, and this also happened out of the blue.
I started keeping Luka away from everyone who came to my house after that... he was either crated or leashed to me when I had visitors.
When my brother came to visit I slowly introduced the two, and Luka seemed to be okay... until my brother was out in the yard one day, and Luka lunged at him when he came up my porch steps. Luckily Luka was tethered at the time, and his teeth missed my brothers leg by inches. This was a serious attack... hackles up, teeth bared... but tail wagging. Not good at all.
So... when Luka was out of his crate, the other dogs and my brother had to be elsewhere (crates and different bedroom) to stay safe.
We managed that as well, because I was determined to make it work.
During all those incidents I had many conversations w/our vet... we put Luka on some pain meds, because he experienced muscle spasms. We thought that his behavior was *maybe* pain-related, because he started to drag his hind legs a lot more and he had more trouble getting around.
It didn't seem to make a difference - a few days later he grabbed my arm twice, HARD, when I tried to put him in his crate.
I still wanted to give him the benefit of the doubt, since I was never scared of him... and since, most of the time, he was such a huge mushball when he was around me, and an absolute sweetheart.
I really loved him, w/all my heart...
We had gone to our vet regularly for check-ups, at his second-to-last visit my vet strongly recommended euthanizing him... she thought he was becoming too unpredictable and dangerous.
I refused and took him back home.
Then over the weekend he had two very bad seizures, and he started to cough up blood... the heartworms were taking their toll on his body at last. He started refusing food and he was very miserable... so, I made another appointment to have him checked out.
He had to be muzzled (for the first time ever) in order for my brother to drive us to the vet, and in order to be examined.
My vet dx'd him w/encephalitis (probably caused by the heartworms) among other things, and after a long and tearful conversation I decided that it was best to let him go... in my head I knew it was the right decision, but in my heart I'm still questioning it every single day.
So, anyway... that is MY story about a "rescue dog" who was only w/me for three months.
I don't know if Boss has any health issues, or if his behavioral issues can be resolved... but, please, don't blame yourself. You took him in, got him vetted, gave him a chance at a better life... it's already more than most people would've done.
There is no telling what he experienced in the past, or if his past still has an impact on him now... but if he's not adoptable, and you can't manage him... you know what you need to do.
Can you first maybe have him evaluated by a behaviorist? Is that an option at all?
I, personally, would not surrender him to a rescue if he's deemed dangerous or unadoptable... as hard as it would be to make that last final vet trip... I would take him myself.
Also, please, be cautious when you interact w/him from now on, and keep him away from the other dogs before things escalate and you, the other dogs, or someone else, gets seriously hurt.
I know what you're going through, and I'm really sorry... I also don't have much advise, but I'm extremely sympathetic to your situation.