What is it you want your son to get from this adventure?
First, its the junior prom , which a limo hardly seems in order, my son went in the family car for Junior Prom. My daughter and friends went in a Limo for Senior Ball. Both of my kids were disappointed in their evening(s). These things are advertised as "magical", but a lot of times the kids are disappointed. I think if they've spent $400 on the night it would even be worse. The girls dress alone can cost $400, then add hair, makeup, jewelry, and flowers for the guy, it's crazy! So who is actually spending the money for this evening? Does your son have a job, and has he been saving to take "that special person", or are mom and dad footing the bill? Is he going with a girlfriend that he knows really well and feels comfortable with? I think that makes a big difference in how much they enjoy the evening and what they learn from this experience. If you as the parent spent all that money on the evening and it was lousy, that is very difficult as well.
My daughter came home from Junior Prom very unhappy, she wouldn't even talk about it when she got home. There is often such a build up of events and they are so not what you thought they would be, and the disappointment is huge.
Another time my son was invited to a co-workers Senior Ball as her date, he was a good looking guy and she needed a date, he went to help her out, but after renting the tux and getting the flowers and dinner etc she took off and hung out with her high school friends, leaving him pretty much alone. He went as a favor to her.
I guess the biggest peice of advice is regarding the relationship he has with the girl he is taking. Is this their first date? If so, thats a lot of pressure and stress. You might be able to walk him through some things that would help them have a great time.
I hope this helps.