OK. I've missed something.
Is the drummer using a floor monitor? Everyone else in ears.
Where is the feedback from?
Is it the mains because you're having to boost the vocals over the drummer?
If so, turn the drummers floor monitor down. If he can't hear it then he'll have to tailor his drum volume to balance that. I suspect this is a mini version of volume wars. Drums loud, turn up monitor, drums play louder, turn up mains to cope. Etc etc.
Turn down his floor monitor.