Unfortunately this breed is very prone to spine injuries in the neck and back. He may have injured himself tripping or he may have tripped due to an injury.
I would not make him do stairs or let him jump up or down off the furniture.
I suggest you have the vet check him as he maybe able to be helped by anti inflammatory meds.
This site has a lot of links to back disease issues in this breed
http://www-hsc.usc.edu/~bjmcmil/back.html
Hope this helps you!
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