Our labs had similar hair loss about 3 weeks ago but not to the point of being bald , just very thin abouve the tail , at first like you we put it down to excessive seasonal molting , but when i asked our vet and mentioned could it be mange she said this was most likely and that thier next dose of stronghols would sort it out as stronghold does worms and skin parasites such as mange mites and fleas ect . It has all cleared up now and hair has resumed almost normal thinkness .
They can pick this mite up from exposure most commonly to areas where fox,s frequent our girls love rolling in what i can only assume is fox poo in the park , and when the weather is wet they dive into any muddy puddle where the fox,s may also have been . This is only a sudgestion you may want to run it by your vet foe his / her opinion .
Other problems as in post above include allergies to food ect , also dogs display a nevouse bitting when stressed about something , be it a change in home situations or new member of the family new dog nest door just some of the things that could be upsetting him causing him to bite himself out of anxiety , i had a cross that did this many years ago we lived in a not to happy household and he picked up on the atmosphere . good luck in getting to the bottom of his problem.
Andrea

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