If you can hear him but he can't hear himself over everything else it is not really a question of volume but balance.
You know all those threads we have here about how this that or the other bass sits in/cuts through/gets lost in the mix and how "my bass sounds great at home but gets lost in rehearsal", it sounds like your guitarist might be getting the same issues - he knows how he wants to sound and can create the right sounds at home, but in rehearsal he can't hear himself as he hears it in his head. Being confused by why he cannot hear what he expects to hear, he just turns it up...
Possibly you could apply some of the thinking that goes on here to your situation, but to solve the guitarists problems...
You know all those threads we have here about how this that or the other bass sits in/cuts through/gets lost in the mix and how "my bass sounds great at home but gets lost in rehearsal", it sounds like your guitarist might be getting the same issues - he knows how he wants to sound and can create the right sounds at home, but in rehearsal he can't hear himself as he hears it in his head. Being confused by why he cannot hear what he expects to hear, he just turns it up...
Possibly you could apply some of the thinking that goes on here to your situation, but to solve the guitarists problems...