Being a former Army person myself, I can say this is a tough one.

I would find out the reasons he wants to join the Marines and go from there. For example, is it out of a deep understanding of what real patriotism is or one of misguided patriotism? Is it out of the perception of the opportunity to join in a MTV/Hollywood movie like created endeavor, or one that is understood for what it is? Things like that.

Also, what I would really try to stress is to get him not to sign any sort of year in advance contract. Get him to think about it up until the very last minute. Things could change before hand that might dissuade him from joining (A buddy of mine signed up for the Marine Corp and at the last minute decided he didn't want to go after all. They blared at him that he signed a contract and his reply was more or less piss off, come and get me. They never did.)

I wouldn't berate him or question his judgements because that could only drive him firmer in his beliefs. He might be looking to "belong" to "something" or to participate. But whatever his reasons are, that might be the key.

Now on the flip side, I would never give my military experience back. It is not something for everyone but the experience is something I am very greatful for. It did allow for the opportunity to do many things - jump out of airplanes with a parachute, be part of a perceived "elite" unit, meeting some very interesting people, going to different places and teaching me great lessons about life and people. (One of the best lessons in life was learned in the military and that was not to be afraid of people in charge because really, they can't hurt you or what is really important to you one bit. They can make you miserable for a short time in a long life, but they can't hurt you.)

On a political side, this is what I would tell him - Bush has shown a true contempt for what the military does by his lying and parsing of the truth. If I had a child or close relative or friend thinking of joining the military I would do all I could to persuade them not to sign up because of how Bush has created the war in Iraq. If he were a truly admirable man he would never have created and played off the fear in a America so he could go to Iraq.