Boyfriend will be joining the Marines

So in about a month my boyfriend and I will have been together for 3 years. We're both 17 (almost 18) and have basically grown through our teenage years together. We're about 2 hours long distance already, but the distance is going to get a lot further- he'll be attending the Citadel next fall and I'll be going to school in Boston. I love him so much but I'm scared for him and I'm worried for our relationship. The Citadel is only the start of everything (forgive me if I screw up I'm still learning about military terms/life) because after he graduates he'll still be giving his life to the marines for another 4-8 years as far as I know. I know it sounds ridiculous that I'm worrying this early because we're teenagers but I don't see us breaking up any time soon. He's my absolute best friend and I can't imagine what he'll be going through. I am so so proud but it's really hard not to cry thinking about next year already. I'm used to the distance but not being able to talk to him for long periods of time seems like it'll be so hard. I'll barely be able to see him either, South Carolina is so far from Boston. Any advice?