Don't bother saying anything at all. For one month, use as much data as you can. If you're in a Wimax location, surf and download with him, use the speech-text function as much as you can, watch a movie or SprintTV while he's around, and then show him your total bill compared to his.
I don't try to sell people who are fans of a particular product any more than I try to get them to change their loyalty from their favorite team because you cannot convince someone to change unless they want to.
If your friend has even more friends who are in the iPhandom, save your breath and just let them be.
Like the dad on the commercial says when the daughter complains about the son adding her friends to his calling circle, "maybe you should have Android-using friends." Not a dig at you or your friends, just a funny borrow from a carrier commercial.