This is a case of switching food to quickly. Switching dog food can be rough on dogs, especially when they are younger. This best way to change is to buy a bag of the new food before they run out of the old kind. Slowly mix a little more of the new food in with each meal. This helps the transition and makes things much easier on their stomach.
Right now, switching him back to his original food would have the same effect on his stomach. Keep on eye on him and just wait out his body's adjustment. If things don't get better, I suggest giving the vet a call. Loose stool is a common effect in dogs when they start a new food.