Browsing this post mostly out of curiosity, but I really like this reply. Dude is tripping over his own feet if the singing is as bad as OP says and the songwriting as good as OP says.

If the band has trouble separating business talks from personal feelings, suggest it as something fun. Maybe next rehearsal ask him to switch roles with you for one song just for fun and see what happens. Maybe he’ll see his song come to life and agree with you