reading after you edit...the beatles route wont work if he is singing terribly on stage. there are many things music listeners will tolerate, bad singing not tolerated well at all.

that said...there may be room for improvement. lessons may help. in ear monitors will almost definitely help. if he can improve to sing well, then he deserves that chance.

when it comes to the actual deed, it is going to be very difficult for him. when someone criticizes musicianship, it doesn't get more personal than voice. the voice is literally his body, guitarists can blame the guitar, the amp, the strings. a vocalist only has their actual body involved. so keep that in mind when you talk about it.

just my opinion, been a part of many bands over the years.