Main character may have been called Jon or Jonathan. He was an American university student who somehow fell into a different world where he may have been the only human, all the other people being taking animals. He discovers that he is some sort of magician who can do unpredictable magic by playing his guitar (a Fender?) and singing songs from his world.
I'm pretty sure I read it at university 30y ago. Ds, who is at university now, is talking about buying a Fender, which has triggered this earworm of a memory.