Another '08 grad here. This thread has some great advice and realistic expectation management, but I thought I would add one more thing that I haven't seen.
I applied out of high school, but did NOT get in. I was pretty upset, but I moved on and started my freshman year at a university. After a few months, I decided I would simply apply again, but after a full year of college I had heard nothing. Finally at the beginning of June I found out that I was accepted, 3 weeks before Plebe Summer! It was a mad dash, but worth it!
Point being, if by some chance your son doesn't get accepted straight out of high school but he knows he truly wants to go, don't give up. Keep applying. As long as you're not 27 on graduation day you can apply until you're blue in the face.
That being said, bet of luck to your son! GO NAVY!