First, if he is joining the Air Force he probably needs a security blanket, but will he when he gets home? 

Secondly, is your sister you, the only difference the passion you have for him?  You with a clear view?  If yes, then believe her.

Another thing is guilt.  Its resonable that you invision him dying away from home alone, no one in his heart.  But that's unreasonable.  His decision to join is independent of your feelings for one another.

And another thing, why commit to military service and not to you?

How bout telling him to come back alive and sound of heart and mind and you'l be his?  That would give him incentive to be careful.  It would also give him time to mature. 

When you're young ... has he ever hit you?  Not all men hit.  Not all men yell at you. 

When you're young it's really vauge, your future, the feeling about living that you want to have some day.  Though its vauge, it's still there.  Its not in your rational mind.  it's there but it won't come into your thoughts.  If you flip a coin it will tell you the answer--but not literally.  If the coin lands on its edge tilted this way, or that way... it will give you the future.  Don't flip a coin, but just like heads falls up so easly, do what's easiest.