8 yr old labdoberman mix hair falling out on back patch turning red and getting hot

Hello. Is the area oozing? What kind of flea prevention do you use and when was the last time it was applied? How long has the red patch been there? Is the dog chewing at the area?

He could be having a reaction to the horse spray. I would try bathing him to wash it off. You can try applying some triple antibiotic ointment to the area, just try to distract him after so he doesn't lick it off. He may also benefit from some Benadryl to help if it itches. If you call your regular vet they should be able to give you a dose for Benadryl over the phone since they will have his weight and medical history. If it's something like a hot spot, that may require oral antibiotics depending on the severity. If you notice that the area becomes larger/spreads to other areas of the body, starts oozing, or becoming very itchy or painful then it would be a good idea to have him examined. I would discontinue using the horse spray.