My love is going to the Marines

I'm 18 and my boyfriend is 20. We have been dating for three years. We fell in love almost instantly when we met. At first we had no idea what the feeling was between us, but we know know we are each other's best friend and are deeply in love with one another. The only thing is he that, despite my feelings toward the military, he wants to serve in the Marines. I told him that I wouldn't be able to handle him gone months at a time and that I wouldn't be able to handle wondering if he would come home or not. But it is a passion--his dream to serve. Maybe he feels he has something to prove, but I love this young man so much it kills me to ever think of him being taken away from me.. I asked how he would feel about an open relationship , but that turned into a big fight.. and basically gave me an ultimatum. I'm either his or I'm not. I can either support him or not. I cannot think about this with out having panic attacks. I get anxiety dreams of him dying somehow and I wake up heartbroken until I hear his voice again... I love him so much but cannot seem to give him a clear answer about what's going to happen after he signs the papers. Can you give me advice on how to deal with this? He knows how I feel and he is so set in taking the steps to joining his dream job. I'm scared shitless.