Thanks for the reply. It's been a couple of months since the teeth extraction so he's not on antibiotics anymore. And yes he did have bloodwork done before the op and everything was fine according to the vet.
It's a very stressful thing taking him to the vet and it takes him a few days to recover from each visit, so really don't want to take him unless absolutely necessary. He's eating well, still very active and his coat and eyes are shiny and healthy looking, so no outward signs of being in pain, just the strong odour from his litter tray. I do get rid of the clumps regularly but the smell still lingers, so could there be other reasons for the smell such as the food he's eating, e.g. a particular brand or flavour?
I might try him on different food too to see if that makes any difference. I tried a water fountain, he wasn't interested at all, he didn't drink from it once! typical...
He also had arthritis in his hips, so not sure if any of the above as well could explain or be a cause?