I mean, with all due respect, try to gather the rest of the band and start ENTHUSIASTICALLY pep him to these sites, other attenuators (as the one at the video link I gave you) and how other players deals with loud tone at reasoneable levels. Show that you all have positive interest and engagement and are comitted to his sound and gear. At least for a while. Otherwise, it can all turn into a sour, and "Grumpy Old Men" whining instead.
Remind him of the following:
1. You're in a cover band, right?
2. How the f*** are you going to land any gig with his volume?
If you play an original band you'll probably are in a different deal. If you're in a cover band you simply want to play gigs, otherwise I don't get it. But most pubs, small clubs (not to speak about weddings) WONT hire you as explained very clearly earlier in this thread by delta7fred. Keep the volume down and land gigs! Keep the voulme up and you wont have any gig at all. At least, not for a cover band. It's as easy as that.
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I did play once with a guitarist who played too low. Yes, ladies and gentlemen, it has happened! And couldn't turn it up even if he was told to. He brought his rack gear, but was a bedroom type and brought his small domestic stereo speakers with him, and his hifi amp. Like, 4 inch, speakers. He was a Lukather freak, and had all these BOSS racks, and had a tremendous sound....in earphones ... !
When turning it up just beyond SPEECH level, it started to squeal and feed back, and the rest of the guys couldn't even play as soft as needed for him to get heard. He refused to alter the patches he had worked YEARS of getting to.
Even if he cranked his stereo with another less distorted and less compressed patch, it was not sufficient. He "played" great though. But that was the largest difference I've seen of a musician that was virtuosic in music, but had absolutely no grip on how to get a good guitar sound. In his bedroom, all sounded ok, playing along with records, but he never ever played with any band. When we got him an amp, and redid his patches to go with the louder volume, he was too shy to let be heard over the drums. Absolutely squashed dynamics and no feeling at all for volume, at the other end of the line.
A very unique specimen ladies and gents.