What's this infection/disease my dog has? ?

Sorry that I can't give pictures but I will try to explain as best as I can (vets can't see him till next week)... He is a sheltie-corgi mix, under his chin there is a large area of what seems to be dried blood and puss, on his back he is loosing clumps of fur, he seems to be developing welts on his stomach too. It started as what seemed to be a scab but when I came back from sleeping over at my friends House my brother informed me that it had grown... It was at first probably a 1 inch square... And now about 4-5 inches long and 3 wide (his whole neck) I used some hydrogen peroxide to treat it as an infection, but no reaction accord (no bubbling) he doesn't seem in pain but he is itching more and more and if you touch it he whines. His sister and my other dog seem fine (no problems with them visibly or behaviorally) could it possibly be a flea borne virus? Any help would help (obviously)