Hi Mary,
I also did some shots of my guitar and found that it's difficult to make an interesting image when placing the entire guitar in the frame. Jiro's post shows how you could do it when going for the entire guitar, but still aim for some interesting details. It might help if you think first about what you (or your son) would find interesting about the guitar and than aim for that in your composition.
For example this is what I did:
About the lighting: I don't know the exact guitar you're talking about, but if your son's guitar has a very shiny finish you might struggle with unwanted reflections. An soft even lighting would help. If you have one, a circular polarising filter would help you cope with the reflections.
Like Colin suggested, you get enough inspiration if you search on google for "Guitar Photography" or something like that.