You can try some aspirin to see if that helps with mobility. Scroll down on this page for cautions and dose info
http://www.petplace.com/drug-library/acetylsalicylic-acid-aspirin/page1.aspx
This could be a knee issue such as a cruciate ligament or it might be a hip or spine problem.
You can see info here on the ligaments
www.marvistavet.com/html/body_ruptured_anterior_cruciate_lig.html
You do need a vet to evaluate him but can probably wait to see your own vet.
I would keep him quiet today. No jumping on or off furniture and no long walks and you might carry him up and down stairs too.
Hope this helps you!
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