Richard Evans wrote:
sighthounds & siberians wrote:

Is he on anything for his arthritis?

Anitinflammatories and pain killers.

Oddly enough, this current problem started not long after he started
the meds, but I've had him back to the vet to have him checked for
physical causes and she never mentioned a possible relation between
the problem and his meds.

Opiate based medications( Codeine, Morphine, Heroin) use constipation.
NSAIDs(Voltaren, Nurofen etc) or Paracetamol have no effect either way.
None of these are likely to be causing your problems.