What to do for a 40lb Rottweiler with loose stools, no other issues?

Before medicating your dog, you should discuss it with your veterinarian. Legally, we are prohibited from providing doses for prescription medication in the absence of a physical exam and a patient-veterinarian-client relationship.
In the meantime, you can submit a stool sample to diagnose intestinal parasites or bacterial overgrowth. Since Max has "simple" diarrhea (no other symptoms), you can try a 24 hour fast followed by a bland diet in small, frequent amounts. Allow small amounts of water or unflavored PediaLyte during this fast. Begin feeding one tablespoon of food every hour and gradually increasing the amount. Feed bland until the stool is normal then transition slowly to his regular diet.
A bland diet can be plain white rice mixed with one of the following: boiled boneless/skinless chicken breasts, cooked/drained ground beef, cottage cheese or cooked/drained ground turkey. Mix ingredients in a 1:1 ratio.
See your veterinarian if the loose stool does not resolve or if Max begins vomiting or develops lethargy, loss of appetite or abdominal pain.