if you guys want to be serious about taking more gigs, consider making a 'feedback session'. where you openly discuss where you want to go and how everyone can improve. this can go sideways when not approached the right way so look up some tips and tricks so people will not feel attacked.

If are all receiving some compliments and some criticisms, maybe he will feel less ganged up upon and singled out.

My best tip is to try to emphasize its you guys vs. stagnant progress problem. not you guys vs singer. unless you've got your mind set on kicking him out of course, maybe first give him the opportunity to improve and work on the feedback from you guys. Playing an instrument is not a skill that's learned in one day, and similarly just becuase he doesn't have it naturally doesn't mean that there's no opportunity for him to become a stage animal in the long run.