how does the military work in the US? Can he like drop out and finish college? Or is there a mandatory time you have to do ?
It's a commitment, you can't just say "my bad", rip up your papers and walk away. There are ways to separate from the military early, but many of those options aren't something you, as a recruit, can request.
When you sign up, you sign up for a predetermined amount of time. Typically between 2 to 6 years. Depends on the branch and other factors.