It could be a number of things, a skin condition, allergies or his diet. Only a vet can tell you what the problem is, and there is a problem that needs to be taken care of so get him to the vet ASAP.
Be a good pet owner .. Take your dog to the Vet!
A dog that is losing fur to the point of balding can be experiencing any number is issues .. From skin allergies and infections to liver and Kidney failure to emotional issues. . You dog NEEDS to be examined by a vet immediately!
All I can add to these answers is that your dog may be under a great deal of stress right now. Stress can cause fur loss, just as in people. Look closely at your dog's life and see how it can be made less stressful.