if you're realistically only gonna be using it on this project, then just grab a Behringer, if not then a ART or Samson should do you just fine tbh.
here's food for thought though, what about just letting him record vocals and guitar at the same time? no need for a headphone mix then, and will get a much more natural performance out of him.
i tend to use a pair of SDC's in a coincident pair between the soundhole/12th fret to capture the guitar, a cardiod LDC to capture the vocals, and a spaced pair of omni mics to get the room ambience. dim the lights and you should get a really natural, relaxed take.