well dang. I can't believe all of the insensative people here. The guy has a problem here. Don't go making him feel worse about something. If you can't offer anything good, then just don't say anything at all.... gee whiz!
MournSanity, The Prom is a big thing to some people, and it's ok for them. For me the prom was just another school party, so I didn't go. My gf (now wife) had a prom a couple of years later that we missed, we sat on her front lawn and talked all night long. On the other hand, my daughter went to her prom and loved it.
But no matter what you decide, just don't be like those people who constantly live in the past and talk about their highschool years. There are so many more things in your life that you have to look forward to. Don't get hung up on some highschool sanctioned party.
Good luck buddy!