You might need to do more extensive testing. I would consult with a specialist .. probably internal med. YOu can find one at :
www.acvim.org
Or...perhaps your vet knows one he/she can refer you to.
Gabapentin works well for pain. Maybe he needs an anti-inflammatory as well as the pain med. YOu said the vet gave him Rimadyl...none for home? A week sounds like a bit of a long time...maybe call the vet and have him seen today and take it from there.
Good luck!