Craig, the other answers all address the possibility that his "singing nowhere near as good" is because he's getting drowned out by the other band members. There's another possiblity as well. If he sings very well by himself, accompanying himself, that doesn't necessarily translate into singing well with other people who are accompanying him. It's a different skill, and needs to be learned. You can't be quite as free with the beat if everyone has to stick together.
So if the problem is that he's straining his voice, then yes, you need to work on the mix. However, if the problem is that everything sounds sloppy, then you all need to work together to get a tighter sound. I don't mean that he has to get rigid to stay on the beat, that doesn't sound good either. But there's a kind of tapping into each others ideas that goes on when a band is really playing well together. Each person has to get to know each other's style, and each person has to value the contribution of every other band member as much as his own. Sometimes that's a problem for prima-donna type lead singers. I know this because I've been one myself before. :)