Hello, as I didn’t get more information I will provide a general reply. Obviously, please feel free to type here any further questions you may have.
Yelping suddenly during any movement is usually typical of some sort of neurological pain. This is usually associated with spinal pain or a nerve pain.
As it happened suddenly, I would say the most likely cause would be a slipped disc, either in his back or in his neck. Other differential diagnosis are not as likely, but they can’t obviously be ruled out at the moment.
Given the potential severity of the problem, the best advice would obviously be to have Rookie checked over for a complete physical examination. If no other problems will be found, treatment with anti-inflammatories and pain killers may be attempted, otherwise further investigations (such as an MRI scan of the neck) may be advised.
Until the appointment, I would advise you to completely rest him (only walking 5-10 minutes twice daily for toilette) and avoid at all costs jumping and stairs, in order to avoid any further injuries.
I hope this makes sense. I appreciate it is a very general reply, therefore please don’t hesitate to contact me at all times if you have any further questions and I’ll be happy to go into it in more detail. Take care.