Ah, bedroom tone strikes again. It can be hard to get guys to listen to the entire band if they've spent a long time playing in isolation and trying to fill the low frequencies themselves. Just ask him to listen to the tone in the context of the band sound, not his tone on it's own. Maybe see if he'll stand across the room from his amp next time you rehearse and listen to everybody, not just himself. Try to stress, as politely as possible, that he's got to have the interest of the sound of the whole band at heart, not just his tone in isolation. You'll probably get more success if the other guitarist works with you on this. You could ask him to sit down and listen while you, the drummer and the other guitarist play through a song. Then get the other guitarist to play through this dude's rig. You'll probably go from something quite pleasant to mush. Hopefully he'll be able to hear the difference. If not... well, there are always other bands.
My current gig is on guitar and, being a bass player, I generally take a little low out and boost the mids a smidge on my guitar amp so our bassist has a bit of room. I don't use a lot of gain either and we're a doomy metal band tuned down to A. It's amazing the amount of guitarists who compliment the tone.