dogman, i SO hear that. i'm cutting myself some slack when i say i only "suck".....
i thought your analysis about a vocal take vs a guitar take is spot on. i think, though, that a large part of it (the pressure) is that vocals are what *sells* the song. if there's something that's UBER critically important that they shine, it's vocals. you can "hide" a crappy musician if you have to.....but if the lead vocals suck (or these days, unless they're 100% perfect), you can forget about it.
in this case, i'd either record the band live, or i'd do my best to make the guy as "at home" in the studio as possible. the studio can be considerably intimidating, what with the headphones and everything. maybe put the singer in the "live room", give him a handheld dynamic and hang a PA system and record his vocals with the PA instead of headphones. sure there'll be plenty of bleed, but sometimes it's "whatever it takes".
Some things I've tried that have helped this situation.
1. Use only 1 can
2. Reduce the volume as much as possible in that can
3. Cut the bass content going to the can
4. Make sure the singer doesn't use a lyric sheet: they gotta know the song
5. Place the mic above their mouth: they tend to sing better looking up
6. Put a cool picture on the wall in front of them: something that makes them more comfortable
p13 made great suggestions. However, as a vocalist I can tell you that it's really hard to sing well if you can't hear yourself. Hell, at home in my bedroom at my little shitty $200 studio setup... if I can't hear myself singing loud and clear it comes out really crappy. Flat, sharp, quiet, loud, shaky, whatever. So it's not even necessarily feeling uncomfortable. Definitely just get it so that he can hear himself really well. I use a virtual metronome playing a bit louder than the music itself so I stay right on beat and I use only one can definitely. And actually, the other big thing is that you *DON'T* want to be able to hear youself over the monitor. Get your settings right and just cut his track from the monitor so all he's got is that music and his pure, natural voice.
'course it never hurts to make him feel extra comfy. Give him a big hug and get him to keep his eyes closed and pretend he's on stage or in a big ball of light or whatever the hell makes him feel all fuzzy and nice inside, right?