Can't help but re-iterate the advice to start from unaccompanied singing -- and if that's not possible in a group (which creates its own 'tune corset' for a less confident voice), then self-recording is a good next step. Note, though, that this isn't to promote hostile self-criticism; rather that it should focus on a couple of areas such as pitching (too low/too high?) or breathing ( broken phrasing).
The bottom line is that improvement is possible and -- IMHO -- that it relates to self-confidence as much as anything.

I was lucky enough to be part of Norman Buchan's Ballads Club when he started it at my secondary school in 1957. Up to 60 of us met every Tuesday after school in Norman's classroom and roared our way through songs such as (Scottish) Johnny Lad, The Barnyards of Delgaty, McGinty's Meal an' Ale; and (American) Michael, Row the Boat Ashore, Pay Me My Money Down.
If there was a focus, it was on energy rather than choral perfection, and always it was inclusive -- everyone could sing!

Please encourage your singer to persist, Will -- after all, he has the proof of his instrumental progress...