dbx is the usual first choice for budget compression. The dbx 166XL is decent for a bit over $200. The 266 is cheaper. There are other brands worth considering, like RNC.
Consider that if his voice is compressed in the monitors he may have trouble singing. If you have a spare channel, use an XLR Y-cable and put his voice into two channels. Use one with the compressor for the audience, and the other without a compressor for the monitors.