I am no vet, but that sure looks like a nasty case of mites to me. Your poor piggie is scratching himself to death. With open wounds like that I would be afraid of infection. I think you need a different vet who actually is savy about guinea pig care to give you ivermectin or revolution, plus perhaps a pain medication and antibiotics to avoid infection.
It really is easy to use ivermectin yourself if you go to guinea lynx and follow their instructions precisely. However, if in doubt, find yourself another vet who will give you what you need. That poor piggie is in pain.
Guinea Lynx :: Mange Mites
Also, all your piggies should be treated. Otherwise they can reinfect each other. You will need 3 or 4 treatment about a week apart. In your case, you may need more treatments to get rid of it completely because it's such a bad case.