Hello,
Sometimes dogs just get intestinal upsets which can be fixed easily, and can be attempted to clear up at home as long as the dog is acting normally otherwise, non-painful, responsive, active, not vomiting and still eating. If he has any of these symptoms or if it doesn't clear up at home, then a vet appointment is necessary, as antibiotics or deworming or just something to settle the GI tract may be needed. With dogs this young, intestinal parasites is a very common reason for this occurance.
For now, don't feed anything for 12 hours, but do allow water if he'll drink. (If not, he can always have some unflavored pedialyte). Then when you do feed, give a bland diet like boiled chicken or boiled hamburger meat with white rice (1:1 ratio). No treats, and don't let him eat grass either. Continue this diet for a couple of days as the stool firms up.
Once firmed up, gradually go back to the regular diet over the course of a few meals.
I wouldn't worry for now if he's acting fine, try this, and if not clearing up in a couple of days, or if he's not acting fine but is showing any of the above symptoms, you'll want to make a vet appointment then.
Best of luck, and let me know if I can help further.
Christine
Certified Veterinary Technician
Licensed veterinary technician (B.S. Mercy College), 10 yrs in animal medicine and training