Should I let my boyfriend join the military?

I understand your worries, but it isn't a matter of what you are willing to let him do. Holding him back from what he wants to do may hurt your relationship in the long run if he starts harboring resentment, wondering what life would be like if he hadn't let you convince him not to join. You two should have a heartfelt discussion about this, weighing the pros and cons, and whatever decision you guys make, try to make the most of it.