Use a dynamic microphone rather than a condenser or ribbon mic. Dynamic microphones are built to take loud sounds whereas condensers and ribbons are for softer, subtler voices. Many recording engineers recommend a 57 or 58. They cost around $100.
Place a pop filter on the mic stand in front of the dynamic mic. This will do two things. It will keep "p" and "s" sounds from standing out and will keep your singer at an even distance from the microphone. This helps his voice fluctuations stay even.
Use a compressor. A compressor is a piece of outboard equipment or an insert on an audio program that, in essence, smashes a sound wave down if it's too loud, and brings it up in volume if it's too quiet. If you have never used a compressor, experiment with the singer and adjust settings until the vocals are no longer over-driven.