Take him to a chiropractor who also works on dogs.

My old dog Wolfgang was big like Thunder and had displasia as well. One more we woke up and he couldn't move it was so bad. We rushed him to emergency and the vet said we either euthanize him on the spot or take him to a chiropractor to see if it helps. After some morphine and pills, we took him to a chiropractor and he came back pup-like.

I'm not kidding. It was amazing.