I'm sorry to hear that Taz is losing weight. I wonder if the vet would be okay with you switching back to a higher protein diet. I know the K/D and low protein is for his kidneys but it might be the reason he isn't keeping weight on.
I was also wondering about a parasite like Catty1 mentioned. Maybe the vet could dispense a Drontal tablet just in case.
I would go ahead with the blood tests if you can swing it because you've already gone through so much with him with this last surgery. If it's diabetes, you can change his diet and maybe he'll need insulin but hopefully you'll have him around for a good long time.
Big prayers going out for beautiful Taz.

E.T.A. Oh yes, as Catlady711 mentioned, maybe it's thyroid. The rapid weight loss made me think of Fatty, the stray cat that lost all that weight and he had a thyroid problem. Thyroid meds are not too expensive if that is what it turns out to be.