Ooof I’ve been there. You need to have a band meeting about it. Start out by saying this is not a personal issue, it’s a band issue. If you (plural) are serious about the band, you need to take steps to make things better as a whole. Things like performances, make sure we’re hitting our cues, not staring at the guitar necks, etc. then broach the topic of singing. My gut tells me he’s not exactly confident in his abilities either. Tell him he has to let go and be engaging and ENTERTAINING, not just fill notes. Have the other guys chime in that you’ve received bad feedback and want to make it better. Then tell him you love the songs but can we explore having a dedicated singer? Make sure he gets a composer rights to the songs in return. Win win.