This link may help you. Although towards the end it refers to certain drugs, it has good information and does recommend a product from Pet Alive that I endorse.
Switch your dog to a raw diet, this will make a very big difference.
And...... Keep an eye on him, Cushings can be serious and has been tied to Diabetes and Pancreatitis in dogs. Any question, seek your vet's advice. I would suggest finding a holistic vet.