My best guess would be that he has probably scratched or bitten himself
there. Guinea pigs get lice and mites. Lice you can see. If you part the
hair and check for very small white cigar shaped insects and see them, it is
lice. You can buy kitten lice shampoo. Dog shampoo can be too strong for
some. It needs to have pyrethins in it to kill the lice. The trick is to
follow the directions well. The shampoo needs to sit on the skin for 3 - 4
minutes, according to the instructions. Then it is to be washed off. Also
a repeat bath in about 10 days will be a good follow up. No shampoo on the
If it is mites, you won't see them. A vet will do a skin scrape and look
under a microscope for them. Then he will prescibe Ivomectrin for them.
Some vets will inject it, I have a prescription to give it orally for mine.