Have you had his thyroid checked? And, bloodwork to check if he is diabetic. Both of those can cause excessive drinking and weight loss. He could also be allergic to the fleas. I had a dog that was allergic to flea bites and the vet gave him steroids, long story short, the steroids induced diabetes. The fleas were brought in by my mothers dog from the groomers. I also have a little long-haired chihuahua that is allergic to environmental things, such as mold and grass, she has to take cortisone pills for her allergies.
Just a few things for you to check on. There are also some good shampoos for itchy skin and dermatitis.
