I would definitely take the dog to the Vet and they may have a simple solution. BUT, there is such a disease called Mange and if the dog has this it can be fatal. Hair loss on the hind area is a symptom. It's important to have the pet checked by a doctor just to be safe.
Allergy to many conditions like food, fleas, lice, chemical, pollens, scabies etc can cause hair loss in dogs. Ringworm infection is another major cause. Autoimmune skin disease and endocrine problems can also cause hair loss. So, your dog needs to be investigated by the vet for accurate diagnosis and proper treatment.
Hair loss in dog not only on tails but from all over the body is due to many irritants that cause itching and licking. There are many irritants that can cause loss of hairs in dogs. These are
1. Flea infestation
2. Allergy to pollens, molds, and foods
4. Lice infestation
7. Cushing's disease
8. Endocrine diseases
9. Autoimmune skin diseases
10. All skin diseases
So, your dog should be examined by the vet for accurate diagnosis and treatment.
The same exact thing happened to my dog. I asked around and people said it could be : Lack of nutrition; feed him brown rice. Or he could be butt scooting on the carpet when you don't know it. Or he could simply just be losing hair, it happens! Dogs are practically human if you think about it. But try to wash him more often to prevent irritation. Good luck!