Boys do change as they grow into men.
I wonder if with regards to the poetry if there has been something happen at school. Has one of his classmates been killed or seriously injured or perhaps one of his friends lose a close family member? It is normal for teens to go through a phase when they try to sort out the meaning of mortality.
as for the military (disclaimer: I served 14 years in the Navy), there could be two reasons.
1. He wants to live out his video game fantasy - in which case he could use a good reality check by talking to a member of the service he wants to join and not one from the recruiter's office. No offense to recruiters, but their job is to upsell the military to make their quota. This could give him some real perspective on what it really means to be in the military. I do know the Navy had a program where young sailors could put in to go back to their hometowns and speak at local high schools to let kids interact with real sailor.
2. He could be tiring of school. Boys tend to be more restless than girls and he may be chafing under the structure of academics and not be interested in doing another 4-5 years of sitting in the classroom all day. The military can be a great option for these folks. it gives them a chance to finish maturing and increasing the odds of success in college for those who were ambivalent about it at 18. It also provides college benefits so that when you graduating you won't be saddled under mounds of debt. If you are smart about saving while in the service it is possible to graduate debt free, but even if you aren't it won't be near what it is for the average student.
If he is serious about the military, he should make sure he signs up for a particular specialty. While I was an officer, I saw far too many of my sailors sold the bill of goods about 'you don't have to decide now. Once you get to your ship you can apprentice to whatever rating you want.' That's not entirely true. The military teaches many wonderful trades and those who who serve in a specialty have a step up over folks who joined purely as infantrymen. Of course there is nothing wrong with going infantry and many folks have made careers out of it, but it's not a skill that translates well to civilian life.
Each of the services also has commissioning programs for motivated enlisted that will pay for their schooling so they can go on to be an officer.
Of course it is a very difficult life and there is the risk of death or injury, but there are several jobs in the civilian world that are more dangerous.
as to the 'shooting range', you should try to have an open and non-judgmental discussion about where the interest in guns came from if no one in your family has firearms. This would be a good time possibly looking into getting him into a firearms safety course if he is serious about it. Far too many people out there are injured by guns because they don't know how to safely take care of or operate them. Just because you don't have guns and tell him you disapprove of them doesn't mean that he isn't exposed to them from his friends.
If you don't feel he's being open with you, perhaps you could look to finding a third party counselor to help you communicate effectively with him. just remember not to be too judgmental or dismiss his plans for the future because they aren't yours - that will cause him to clam up.
Good luck.