I can only echo what everyone is saying, please don't give up hope. Equally important, please don't give up faith in yourself that you are doing everything you possibly can for Canyon.
Eaglflyt made a very good point when she said "The muscle spasms can come in *attacks* and they are terribly painful. Also, his guarding his neck with being tense can trigger the spasms from that activity, too." For the first several years after I broke my back (and a few years after I broke my skull...and really, I am rather graceful, both were accidents that happend to me as opposed to clutziness) my back would spasm so badly that you'd think an alien was trying to get out. After my head injury there were prolonged periods where if I turned my head in either direction, it would stay that way for days. Even as a child, I was able to realize that these things were as a result of my accidents. Canyon doesn't reason that way, he only knows he hurts, not why he hurts. His cries could be as much frustration as they are pain. At least I could ask for heat packs (I hated the medication because the best solution was rest so they just kept me asleep) when I was in pain, Canyon can't.
You are doing an amazing job thinking for him and trying to anticipate what he needs and getting him every possible test and care. It may just, as you've heard already, take a lot of time for Canyon to come out of this. That said, when our pups hurt, it does seem as though time lengthens dramatically. I truly believe that the day is coming when Canyon will stand up, give himself a mightly full body shake out and be back to his true self.