If it is not from a diet change, two common causes are a blocked bile duct and exocrine pancreatic insufficiency.
Bile is the yellow fluid that is produced by the liver and stored by the gallbladder. When it is time for digestion, the liver contracts around the gallbladder and bile flows down the bile duct into the small intestines.Bile carries a variety of the body's toxins that are then dumped safely into the gastrointestinal tract and ultimately eliminated in stool. Bile also assists in the absorption of dietary fats, fat-soluble vitamins, and other desirable substances. When the bile duct is blocked, the yellow pigmented bile is not in the digested food material so the feces loses some, or a lot, of the brown color.
This would have to be diagnosed with blood work. If the bile duct is blocked, he could be put on an oral medication to remove the sludge from the bile duct.
Exocrine pancreatic insuffiency is the inability of the pancreas to make the three necessary enzymes to digest food. Because the food is not properly broken down, the feces comes out in colors ranging from tan to white. Blood work is needed to diagnose this one as well.
www.VeterinaryPartner.com/Content.plx?P=A&A=1627 EPI you can read more about it here
Just so you know, garlic can be toxic to dogs if given in high amounts. It causes red blood cell destruction. Anything over 12 grams a day would be toxic to an 11 pounds dog per text books. But we can not be sure of the exact amount that would cause toxicosis in your dog. I would not feed it to my own dogs.
I hope this helps.