Does he digest it well, or does it give him diarrhea?
I think it's ok to indulge your cat in this treat, as long as you don't over-do it, and as long as your cat doesn't have digestive upset from it. Cats are not built to digest pineapple -they are carnivores- but if you are not seeing a problem, then the occasional treat is ok! There's nothing toxic about the pineapple for cats.
I hope this was helpful. Please let me know if you have more questions. I would be happy to talk further with you.