If you're sure the bald spot isn't mange then you can go to pet stores or Walmart and get some Hotspot Skin Remedy for dogs that you apply to the dogs hotspot overtime.
The Hotspot treatment for the dogs hair and skin will help the hair grow back eventually.
If the bald spots are due to mange then you'll need to treat the mange.
A variety of conditions such as ringworm fungus, pests like mange mites, and bacteria can cause an infection that leads to hair loss.
So it would be a good idea to get the dog checked out by a vet just to be sure.
You can also give your dog some omega-3 and omega-6 supplements which can help keep the dogs coat healthy and prevent bald spots if it's not due to some viral infection.