I know you said for these gigs it's a vocal-only PA so that complicates things really. It's a difficult trick to be loud enough to cover the room whilst not being deafening on stage.
If your amp was only an onstage monitor then I'd have said go for it. Or, more sensibly, turn down on stage so he can hear you from where he is or set your amp up to fire sideways across the stage. As your amp has to cover the room then all bets are off, maybe try putting your stack in a corner facing 45o across the stage? That way the guitarist should get more of you, but then so will the main vocal mic....
The real answer is to buy him a wedge or an IEM system, but that's not always practical and always expensive.
Just try stuff until you fix it, it's only rock n roll.