I encountered an almost identical problem. I first thought allergies or mange, but could rule out allergies because there were no significant household changes, or diet.
Dog constantly scratching, losing LOTS of hair, but overall no significant changes in personality.
Turns out it was a unique type of bacterial infection on the dogs skin, which caused much hair to fall out and not regrow. This got progressively worse and the dog had bald spots that would grow larger over time.
Treatment was antibiotics, and a daily bath with a prescription shampoo that had a special procedure for use.
After about 3 weeks it seemed that the fur was filling in. In 3 months the dog looked normal again.
Take the dog to the vet. It's the best possible choice you can make, and should yield the best results.
Don't wait too long because if it is a bacterial infection, remember that bacteria multiply on an exponential basis and the problem will compound rather quickly.