What you do is, all of you need to keep a half-step up or down from him (except him, obvs. or he’d run out of kstack and BSOD, and using that as the basis for a kick-out might look bad), and then confront him afterwards with his obvious tone-deafness.

Now, if he’s not tone-deaf, he’ll self-correct and y’all’ll have to switch to and from the actual notes as he does (I recommend a gradual sinusoid around the usual pitch—it helps, if signing, to pretend you’re an 80-year-old contraltos with a helluva vibrato). That’ll take practice and your keyboardist keeping a spare elbow on pitch bend, but it’ll only boost your own, personal musicalities to get good at it, giving you a higher horse to spit at your lead singer from (or whatever people are supposed to do with their high horses).

The audience is likely to notice y’all modulating like crazy people, but pretend it’s intentional and part of your unique sound, and it was Frontman’s idea anyway, but you’re getting rid of him so it’ll be obscenely normal/non-unique next time.

Or just tell him. Less fun tho’.