The best way to get a guitarist with a Twin Reverb to behave is to use the tilt back legs to aim the amp right at his head.
Seriously.
You don't have to say anything. He'll figure out what is too loud on his own.
The last time I ran into a situation that threatened to get too loud I told the guitarist (band leader/front man) that I really liked his full stack...
and if he turned it up I had my BX1500 bridged for 900 watts and I wasn't afraid to use it.
Scared him so bad I had to talk him into turning up some so the drummer didn't drown him out.
Seriously, preserve your hearing.
You'll need it later.
Seriously.
You don't have to say anything. He'll figure out what is too loud on his own.

The last time I ran into a situation that threatened to get too loud I told the guitarist (band leader/front man) that I really liked his full stack...
and if he turned it up I had my BX1500 bridged for 900 watts and I wasn't afraid to use it.
Scared him so bad I had to talk him into turning up some so the drummer didn't drown him out.
Seriously, preserve your hearing.
You'll need it later.