It sounds like he has mange mite to me, best take him to the vet and they will do a skin scraping to check for mites also if hes scratching a lot then this is probably what it is but I can't tell for sure because you didnt say where abouts the hair loss was on your dog? don't worry if its mange its fairly easy to treat. Other things it could be include a reaction to flea bites or the thyroid glands not working properly, theres also a type of alopecia that dogs can suffer with. Hope this helped :)
