Dog hair loss is a natural process that dogs go through especially when they are growing. This mostly happens to dogs which are double coated. The hair loss can also be due to a medical condition. It would be best to take your dog to the vet just to ensure that there is nothing wrong with it.
The dog could have mange. You need to take him to the vet as soon as you possibly can because the sooner you catch mange, the better the chances are of getting rid of it successfully. There are two kinds of mange, and only your vet can determine what kind a dog has. The treatments are different so it's extremely important that you see your vet. Do not try over the counter or home remedies as they don't work, and you need to start correct treatment right away.
Well, even during a heavy shedding season, the dog wouldn't be left with bald spots. Inspect the skin around the bald patches carefully, using good light and parting the fur that's left with your fingers. Look for any discoloration, flakes, insect bite marks, etc. A good web site to check is petmd.com if you can identify more symptoms. I think a vet visit is in your future.
Several things can cause this. Allergies, mange, hypothyroidism, liver issues, and the list goes on. Please call your vet and have your dog check out. Everything I mentioned is treatable, but if you don't treat them you dog can become very uncomfortable or sick.