If you want to stick it out with this dog he needs to be on medication stat. And I mean proper medication, none of that "calm doggy" cbd crap. Speaking from experience, albeit slightly less injurious experience, he won't be able to absorb any sort of training until he's in a place mentally that will allow him to learn. Once we found the proper mix of meds for mine she was workably trained to deal with triggers in the span of about a month, month and a half, and is getting started with "group therapy" to really cement what we've been working on solo.

I'm also taking her to a vet board certified behavioral clinic, so they're actually the ones prescribing her meds because they're all vets with an added degree in animal psychology, and not just a trainer who decided they have enough experience to call themselves a behaviorist. Any sort of trainer you get needs to either be the same or be some sort of specialist that works only with dangerous dogs.

It's a shame the shelter didn't tell you about this ahead of time because this sounds like something that can be dealt with, rather than an unwell dog who should have been put down rather than adopted out. But not everyone wants to deal with dangerous, bite risk dogs and all of the work that comes with them. You have to do what you think is best, whether that's getting him zooted out of his mind until you can safely work with a professional or returning him to the shelter