The term "my dog is losing his hair on his back" refers to a condition known as canine alopecia, which is characterized by hair loss in dogs. Canine alopecia can be a localized condition, affecting only a specific area of the dog's body, or it can be generalized, affecting the entire body.
There are a number of different causes of canine alopecia, including allergies, hormonal imbalances, skin infections, and parasitic infestations. In some cases, canine alopecia can also be a sign of a more serious underlying medical condition, such as cancer or Cushing's disease.