So I have a player who is a druid. He says that, while using whirlwind, creatures can't damage him. Is that the case?
No.
Nothing in the whirlwind ability suggests invulnerability. Furthermore, no other effect, not even the other ways of transforming yourself into air, provides complete immunity to attacks.
Ask him to to tell you where in the rules it says that.
When he can't find it (because it doesn't exist) you can safely say no.
Your player is hung up on the concept of a whirlwind. The concept seems to suggest that physical weapons and such would pass right through it.
Sadly (for your player), game effects do only what they explicitly state that they do.
I ran into the same problem with gaseous form a while back. Against PCs armed with magic weapons, changing into gaseous form (without a handy grate within 5 feet to disappear through) turned out to be the absolute worst strategy my coolio vampire could have chosen. It seemed so appropriate at the time, but the RAW said differently.
Live and learn.
