If you don't want a relationship with him, tell him that your night together was a mistake and put your foot down about not wanting to be in a relationship. Ignore anything he tries to say to you to try and get you to change your mind about it, and if he tries to make any more advances I'd think about changing room mates.

If his plans were to join the military to begin with, it'd be okay for him to move out, but maybe get some other potential room mates set up in case things turn sour.