we had some issues in my band about 8 months ago. our singer was just lazy. it was obvious the only time he really did any singing was at rehearsal. but he maintained the attitude that he was the boobies. he's remarkably talented, but if you don't put in the work, who cares? recording is nice, but playing shows is the bread and butter. and our singer was getting tired three songs into the set. then he'd start turning his head away from the mic because he knew he couldn't hit the notes and i guess he figured if he was acting like he was really wailing, people might think he was?
we had a sit down with him and straight told him he was stinking it up. he made the choice to put in more better work and so far it's working.
i'd advise you to sit him down and talk to him about it. if it's a question of better work ethic, more intense practice, whatever. just tell him you've had grumblings and he needs to step it up.
either he will, or he won't. the band might get set back a bit, but if he buckles down and gets to work then there ya go. and if he doesn't, you've laid out the options for him.
good luck!