Flea bites, irritated skin or fly bites. It's most likely flea bites because they bite and irritate the dogs skin and make dogs dig and scratch. So you need flea killer to deal with this
My Dog Is Scratching Losing Hair, Could It Be Sand Fleas And How Can You Tell?
It could be many parasites, and you should seek vet help. Mange comes to mind and if the topical kind is caught in time it may not be fatal. www.google.com