All right, so I have a pretty good hunch... He knows exactly what you think about the military, and that's why he's not contacting you. He's embarrassed and/or afraid of talking to you. Why? Because he KNOWS that you will be upset with his decision. However, don't allow him to avoid you any more. Go to his house, find him when he's hanging out somewhere, whatever - just don't be a stalker. :)
Anyway, talk to him like you normally would at first, tell him you haven't seen him for awhile. When you ask him what's new, he might tell you what's going on with the military, or he might try to avoid it. If he tries to avoid the subject, casually ask him if he's going into the army, because you thought something on his facebook. Don't sound accusing or angry, just sound curious. Be cool about it. Don't be an asshole and demand that he tell you what the hell he's doing. Don't make evident your extreme disapproval and do NOT tell him that his decision is stupid. I can almost guarantee that your friendship will suffer if not end if you attack him.
Let him know that you support him, but you're surprised at his decision. Ask him what made him decide to go down that path. Be patient and listen to him.
Good luck, I wish you the best.
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"In a world we need to share;
it's not enough to stand and stare." --Pink Floyd