A dog with diarrhea may be treated through a dietary cleanse, behavior modification, a dietary change or by a veterinarian. These treatments should be applied in that order unless there is reason to believe the diarrhea is caused by medication or disease. In these cases, call the veterinarian.
First cleanse the dog's system by withholding food for 12 to 24 hours, ensuring he has plenty of water. When the diarrhea stops, feed him small portions of plain boiled rice and either boiled hamburger or chicken. Add in his regular food over the next few days. Meanwhile, make sure he's not scavenging. If he is, the items he's scavenging need to be secured, and he may need behavior modification training.
If he still has diarrhea, eliminate dairy from his diet, and feed him a carefully measured diet in three equal portions throughout the day. A pet store can recommend a good dog food that may help. If the diarrhea continues for three weeks or he has other symptoms, call the veterinarian.