Also, if you are looking to build something for him: did you consider the wedge horns? I would thing a W6 would be fine, it would be so close to him. If you insist on bigger, a W8 would do more than blow his head off. He'd be blown out of his socks. A W6, see if you can fumble a top hat in there if it must be at ear height. Separate EQ for the drums monitor seems in order. What Bruce says: roll off everything below 120Hz, above 5kHz or an even tighter range. The voice should jump out. A 31 band EQ or digital EQ would allow to suppress specific feedback frequencies.
Standalone the vocal monitor should sound honky. The wedge horns sure do. When the band is playing music, they are golden for singers.
Having the same microphone type for all singers in the band helps. Even more if all are on the same channel from your desk, and on a single EQ.