The lab is prone to seborrhea, hot spots, allergies (Atopy), and hypothyroidism. All of which could cause hair loss , sores, or scratching. Hot spots would cause a sore especially if the dog scratches a lot. Atopy and seborrhea can also leave sores.
Bacterial, fungal and yeast problems are other causes for hair loss and sores but to determine any of these a vet would have to do a skin scraping.
Seborrhea would need a special shampoo to treat, hot spots would require you to clean the wound frequently with antibacterial soap and apply a triple antibiotic ointment on it, Allergies or Atopy would also require medication .
You also must consider that maybe he did get hurt on something and a scab was starting to form and when you brushed him the scab and hair came out. Try treating it with an antibacterial oinment and see if it clears up.
As you can see, skin problems for the Lab are many and the best thing to do is to have the dog examined to determine the cause if treating it at home does not work.
Oh, you did not mention that in your original post. I'll go ahead and opt out, maybe one of our other experts can give you some advice. Best of luck to you.