Dog woke up with pain and lameness in back legs

Hey all I need some advice, this morning we woke up and one of my dogs started helping in pain and his back legs didn't work well. He could walk a little but thats it.

Took him the vet right away and they said they see this alot with active small dogs and then gave him an exam. They gave him morphine and something else immediately for the pain and gave us three types of pain killers for him. Cerenia 10mg/ml (Maropitant) Buprenorphine 0.6mg/ml Carprofen Chewable 75mg Gabapentin 100mg Capsule Methocarbamol 500mg

They told us he needs a month of rest basically, so he's been resting all day and I tried taking him out and his back legs are not responsive at all and his toes curl back. They told us to go to a neurologist and the closest one is 2hours away and closed already.

Realistically what do you think might be going on?

My dogs a jack Russel mix and is 3 years old weighs 17 lbs very active. My dogs are inside dogs but they go out serval times a day before and after work.