You could get a spot on from your vet called advocate, that gives 4 weeks treatment. Ear mites live a cycle over i *think* 4 weeks which is why treatment is extended as you have to wait for the immature eggs to develop into adult mites which the drops will kill.
Damp cotton wool (warm water) is fine for cleaning his ears, dont be tempted to use cotton buds as they can cause alot of damage