Any chance it got bit by an animal or possibly insect bite? Or is there a cut there that could now be infected? If you don't think it's any of these, rabbit syphilis, possibly mites, or myxomatosis(depending on if you live somewhere that has this disease) are some possibilities. Check your bun over for any other signs of swelling or crustiness. MediRabbit (rabbit syphilis) MediRabbit (mange mites in rabbits) MediRabbit (myxomatosis in rabbits)
If you can take him to a vet that would be the best option. You can get Pen G at a feed store, but it must be injected, dosage based on weight, and giving it without knowing what he has may not be your best option.