Guinea pigs regularly (unfortunately) come from pet shops and private homes already carrying a variety of skin parasites.
Among these are ringworm and ear mites.
If you are feeding this little guy a good diet with a vitamin C supplement (GPs can also develop scurvy from vitamin definciency), AND you have him on an inert bedding (most GPs are allergic to cedar and some are showing problems with pine), then I do strongly recommend that you have him examined by a GP-familiar veterinarian who can diagnose this ear and get you safe treatment(s) for it right away.
Ringworm is contagious to people and other pets, and ear mites can be transmitted to other animals in your home.
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