First, I commend you for raising a fine young man who is looking to serve others. I was in a similar predicament years ago, although I came from a Navy family and thought I'd go that route. But I decided to talk to the four Dept of Defense recruiters (Army, Navy, Air Force, Marines) and listen to their pitches. (I didn't talk to the Coast Guard back then, but I'd certainly recommend your son check it out. Its already important mission is IMHO even more important in this terrorism environment.)
I ended up choosing the Marines and really enjoyed my service. But I can also say that I ran into some awesome people in all branches. Your son's desires will likely decide for him. (For instance, technical training might push him toward the AF or Navy, while physical challenges/fitness/leadership may push him toward the Marines or Army).
Since neither of you have any preconceptions, I'd recommend going to each recruiter's office with him, or calling them on the phone with him on another line. (Only if he's comfortable with this approach.)
I've probably blabbed enough. :-) Good luck on his decision, and below I included a nice one-stop shopping link for more info. Of course check out each individual service's sites as well.
Oops. Meant to say that I pinged some veterans from various branches and active troop supporters to perhaps offer other perspectives. Cheers!
Thanks for your input.
Another poster said I had highjacked this thread which was not my intent. For that I apologize.
God bless you.