I assume he'll be using headphones for monitoring? If so, push his vocal in the mix loud enough so he'll back off a bit. If he complains, tell him you really can't get him any softer in the monitor mix without adversely affecting the recording level (will he really know?). Ask him to hold the mic back a bit and tell him when its "just right". If he still works the mic too close, push his vocal a tad bit more in the monitor mix and keep giving him positive words of encouragement when he's where he needs to be. More than half the work in getting a good performance involves a bit of "therapeutic" skill (manipulation).
