True, generally you don't want to abruptly change food, but on the other hand, if the cat already has a very upset tummy on a given food that he has been eating for awhile, I'm not sure that you're going to make it worse :/ May as well just switch it, especially since Whiskas and Purina are pretty lousy brands to begin with. One thing you shouldn't do though is switch to a bunch of new foods in rapid succession looking for one that works. Each new food will take some time (weeks) to evaluate. So it could be a long drawn out process. You should probably just start with trying a hyperallergenic formula, one that will have completely different ingredients across the board from what you are feeding now. If that "works" then you can investigate getting him something better once you know it is the food.
But definitely follow up with parasite testing though. Having a really really smelly poo is a tell-tale sign of something like Giardia or TF, so that's probably what it is. If your vet is not being helpful in getting this taken care of (which sounds like it is the case) you may need to find a different, more helpful and less clueless vet.
_________________ The little monsters 3
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