Seeing your note in your SS about trying the gabapentin, I think that may be worth doing. If the buprenorphine isn't quite controlling his pain, the small boost from the gabapentin may be enough to get it under control. Hopefully that will make his appetite better.
Start with the lowest dose your vet has recommended for the gabapentin first and see how he tolerates it. Most cats do fine on it, but some are sensitive (I have one who can barely walk on 50mg... and another who's taken 300mg with no noticeable side effects). It usually takes 2-4 hours to kick in fully.
Hope it helps