While Scott's method will work, and has been done many times successfully-
if you plan to do more with the tracks, it may be better to let him play it and then sing to it after; cleaner tracks (no bleedover) and only one performance at a time needs to be 'good'... not a perfect guitar track and vocal track at same time. Sometimes it helps to focus on one at a time.

However, some people want to play/sing it all at once and are capable of pulling it off, so you'll have to select from your options there. Most times I try to record both at the same time the guitar player is moving a lot while singing and the guitar track does not get a consistent signal.

I do not work here, but the benefits are still awesome
Make your sound your own!