Man, did I post this and not remember it?
That is almost word-for-word my situation...
My guy has tried in-ears (some brand I did not recognize...) and couldn't make them work with the rehearsal room PA so that's gone nowhere...
He has used some kind of TC pitch-correction pedal which might have helped. Not doing that any more either.
He keeps threatening to retire. If he continues to deteriorate, the marketplace will make that decision for him.
So far, the gigs keep coming in and the checks continue to clear. The setlist is reasonably fun and I've been learning a lot playing with horns.
But frankly it's just the money that keeps me going with this project.
I'm 63 and I started out in music as a singer. My voice was pretty solid for most of my young adult life and I never worried about intonation much because my ears were good and I was singing so much that everything I needed to hit those pitches was in good working condition.
Then I got out of the music business entirely and went to work for about 25 years.
When I decided to get back into music, one of the first things I realized was that - not to put too fine a point on it - my voice had gone to hell in a handbasket. What a lot of good singers don't realize is that, as you age, the muscles you use to sing weaken. Intonation in the throat is just as much about muscular precision as intonation on a fretless is about being able to put your fingers in exactly the right spot, every single time. That's not as easy to do in either case when those muscles are atrophied.
It might be a problem with a singer's hearing, but if he's saying that he knows it, he may be able to address this by doing some physical toning in those areas. Most singers who have been doing this for years might not want to hear this, though, and might not want to do it, because they never had to before. But, here's the thing. Even if you sing frequently, if you're not keeping on top of this, it will still deteriorate. I've had to work very hard over about five years to get my voice back to about 90% of what it used to be.
When I was in my mid-30's I joined a group as lead singer. They were the most accomplished group of musicians I'll ever work with, and the bar with that bunch was ridiculously high. After listening to some rehearsals I realized I had to seriously up my game, even though I'd never felt like I had problems in the past. As lead singer I had to be very active on the stage, which meant I needed a lot more breath control to be able to move about like that, sing, and not get winded while continuing to put those notes exactly where I wanted them. So I started a series of workouts where I would sing during strenuous activity.
I'm sharing all this not to make this post all about me, but to help people to realize that their singers may not know what to do to fix these sorts of problems as they age. If they are natural vocalists who have not taken many lessons, they may not have a very deep bag of tricks for doing even basic things like vocal warm-ups. There's little awareness among many vocalists i've worked with of the need for these things.
I'm not going to suggest what you all might say to them, but these are some things you might keep in mind as you think about how that conversation should go. Gently encouraging them to step up the private practice and warm-ups might go a long way toward solving these problems, but they have to feel like the work is worth the rewards.
Oh, also... it's not going to fix itself in a week.