I would take him to the vet to be safe. It could be anything... He could be annoyed by fleas and scratching, mites etc..
There is many possibilities I'm sure Jbun will come along and give you great advice
With the location of the fur loss and the fact that your piggie had ringworm recently, I would suspect that ringworm is causing the fur loss. Though mites or other skin condition are possible as well.