Sometimes dogs do get a little loose stool when you change their foods. It shouldn't last more than a few days. If it's REAL diarrhea (ie going several times a day and loosing a lot of water) then you need to do something about it because that can cause dehydration. If it's just a couple of loose stools a day, keep an eye on him and it should go away on its own. Make sure your dog has lots of water.