Hi There, Its an easy solution tell him to sing up or ship out.... However, prior to that he may of course have a problem remembering lyrics if so get him a music stand (Note the stand in a picture of one of the bands I was in, this by the way was a top class band but a member had the trouble mentioned)that he can glance at.... I have also known band members who don't think they can sing, some of them were right,others quite wrong. so you have to determine if you are a fun band or want to achieve things and take it from there. If he has a problem I am sure he will appreciate any help you offer.
Regards Fenderphil