Hello I am Dr. Joey. Thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 15 years of experience. I look forward to working with you.
There are certainly several potential causes for the behavior you describe. My top suspicion would be Valentine has a neck or back injury which is causing pain. This is our most common cause of difficulty moving up/down and crying out at random. This would be an emergency if he is losing the ability to coordinate his hind end. Then you need to take him in to be seen right now to prevent the progression.
Alternatively, he could be having joint issues (hips, other joints) which can come from many problems ranging from inflammation/infection to dysplasia. An exam by your vet should be able to isolate pain and then an X-ray may help elucidate its cause better.
I really wish I could provide you with an exact diagnosis, but I too would need to perform an exam and some testing (at least radiographs or X-rays) to know what is going on. So, the absolute best next step is for him to be examined by his veterinarian.
I am at a point I need to know what questions you have. We can continue our dialogue in this setting.
I hope that the information I provided has been helpful.
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