I and my GP Salvatore have been through the exact same thing in the past two weeks. My vet agrees with me and the others here, that it is a fungus infection, but probably not ringworm. We discussed giving him itrafungol (we do have that here), but because of the expense of the medicine, my vet suggested that I simply continue with what I am doing, which is bathing Sal and, while he is still damp, rubbing in a liquid mixture of water, hypoallergenic lotion (for the dryness) and clotrimazole (for the fungus). If I do this on a regular basis Sal's hair grows back and the flakiness stops and he has no itching. We don't have a great deal of the guinea pig medicines over here that you have there, nor do we have the cavy savvy doctors.
Regardless of what I say, you need to follow the suggestion others here have mentioned of taking the little guy to a veterinarian. I hope he gets better real soon.