It could also be a fungal infection, which is more common in dogs with floppy ears. The ear fold keeps in more moisture than in pointed-eared dogs. It would be a good idea to have your vet take a look so you're not blindly throwing things at the problem that may not work, or may make the situation worse. It's also a good idea to occasionally check for, and remove, any ear wax that is visible. We swab ears with a Q-tip about once a month.
It might be allergies but it might be ear mites. Smell his ear if it stinks he has ear mites. Try treating him for them and if the sores continue to appear contact your vet.