I would call your regular vet or a nearby vet and ask them to suggest you to an exotic pet vet. Most vets that handle reptiles are usually willing to handle small animals as well. Especially with something simple like this. He clearly has a really bad inner ear infection.
Print this out and bring it to the vets with you
http://www.rmca.org/Articles/dchart.htm and highlight mentioned medication. This should make it really easy on the vet if they're not feeling 100% on dealing with a rat but without the medication from a vet he might not make it.