A keyboard player who plays too quietly and you want to turn him up? What's wrong with you??? You should thank him and offer to buy him a lap dance after every gig!
You could try putting him in the monitors if you have separate mixes. If not, then maybe suggest to him that he put his amp somewhere where he can't hear it as well and turn it up so you guys can hear it but it's not blowing him out. He's probably trying to get a good balance with you guys but it's too much in his face and the rest of your amps are harder for him to hear. So he compensates by turning too far down.
I don't think you guys should turn down if you're not getting complaints, and I don't think the singer should work his volume in the PA. Working the volume is cool if you're a dedicated soundman, but not if you're the singer or musician. So see if he'll go for the moving the amp idea.