We have a similar problem, not the feedback, but being too loud in a small pub gig, I have raised the subject and asked if he could play quieter, but he just says he enjoys bashing hell out of the kit (he regularly trashes his 300 quid cymbals and at the end of gigs it's like a wood yard round his feet), in all other respects he's an ideal band member, decent drummers who don't cause problems within the band don't grow on trees so we just accept it