Yep, I'm going to bring my Zoom out to the next session and maybe we can get some time to review the results afterwards over some beers. We've dashed off quickly afterwards before with everyone so damn busy it seems.
I suspect the problem lies in his learning guitar at home playing alone. Guitar tones that "work" well at home routinely don't sound good in a full band mix. He's uploaded some demoes online with simple drum tracks/bass lines, so possibly that's why he thinks he needs such a thick stew of effects/gain. His stuff is all blues/blues-rock, so certainly doesn't warrant the tons of saturated gain he slathers on. I can't rule out that he might be compensating a bit for his playing with all the distortion, consciously or not.
Not knowing the people involved at all, it's difficult to say. I would say that your assessment is most likely spot on. If he's open to improving the music/the band, it shouldn't be a hard sell if presented constructively. Maybe framing it in the context of his songs being diluted/muddied up, sometimes less is more approach?