I have a three month old chihuahua and i just got him two days ago?

Stool eating is a normal but disgusting thing dogs do. All dogs do it sometimes and some dogs do it all of the time. Most pet strores sell stuff you can put on their food so they won't do it. Two brands that come to mind are For-Bid and De-Ter. There are other brands, too. You can probably buy it cheaper online. Try www.kvpet.com or www.jefferspet.com