Why does my dog eat cat poop?

Ok, I have a little chiweenie puppy around a year and a half. We have had him for around 9/10 months and we try to stop him but he wont stop eating it! We do feed him enough, and get him wormed, but he has been doing it for a while and it is not nice. I have two kittens, and one of them is not litter trained so he goes around the house (Yuck!) He is learning though. So what can I do? and how can I help?