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Looks like fungus to me, you can read about it here: https://www.guinealynx.info/fungus.html
Did you read the link provided? Antibiotics are generally not needed for fungus.
Guinea Lynx also has a list of safe medications.
If you are treating one pig for mites, then all the pigs should be treated. There should be at least three treatments each a week apart for each pig.
Two weeks apart is not close enough. At the very longest is should be 10 days.
If Prince was with the others at all, or even in the same room as the others, there's a good chance they all have mites.
You don't have to have the vet treat them all. You can do it yourself.