Hey there, fellow member of the Chris Club.
Yup, the compressor is the device that will tame that wild singer's tracks, even without the conciderable benefits of good mic use and vocal technique. Well, almost.
I would not buy a mono unit as you'll need two for recording stereo sources. I would only buy a 4 channel unit if you foresee the need to compress 4 channels at the same time while recording. Do some research and get the best one you can afford because you will use it on a lot of things.
Having a singer that is too varied however, will still sound like crap even with a compressor. When it has to work too hard to bring a loud part down it changes the sound too much and sounds...welll, compressed. Try having your singer back a few inches away from the mic for the loud parts. Show him the meters and let him watch them while he sings so he can get a sense of what he needs to do to get the levels more acceptable.
Believe me, pro singers have to learn how to do that to: no device can make up for a singers lack of mic technique. Until he gets that down a little, even a compressor won't sound as good as you want it to.
So your singer has something to practice while you research the right compressor for your needs.
Chris