Did your vet do a scraping at all? If it's all over his body now and scabby, you may want to take him back to the vet (a different vet maybe). It's probably not sarcoptic mange, or you most likely would've got it by now
but there are other things that can cause yicky itchy skin aside from demodex, including allergies. You also want to watch he doesn't get a secondary infection...how bad is it? If it's fairly severe, it may be worth your while to put him on ivermectin (through the vet).
good luck, hope your pup feels better soon