Their poop may smell because they don't have enough bacteris to break stuff down yet. Try giving him a few teaspoons of yogurt-plain no fruit or anything. I'm sure you already have seen the vet but I hope that helps! The diet is a big thing. When buying/ bringing a new cat/kiten into your home you should always ask what they are feeding it ...well they should tell you you shouldn't need to ask. Water also makes a difference. Well water...or city water or bottled. Switching that could give them diarrhea. Just moving into your home could do it. It's good you are taking it to the vet though. They can get dehydrated really fast. Also have the vet check the stool for worms! Good luck! Let us know how your new baby is doing!