I really love this guy, and i need some help?

Okay so, theirs this guy Leon. I've known him since i was 11 he was 15 at the time. I'm 17 now and he's 21. We live in different states, so i never met him in person. This guy has been through the ups and downs in my life. He's always been there for me, as i am there for him. Everytime i get a boyfriend he disappears. He doesn't call me no more, email me or even text. It takes about 3-4 months and he magically reappears in my life. He asks me how life is going and if i'm with anyone. Which i'm not dating anyone once he comes back. But about a week before he reappears back in my life something bad happened. Its as if he knew and thats why he came back. We talk for hours non stop, and talk about seeing each other in person. He tells me he loves me everyday and that i'm gorgeous. He's very into the marines and wants to be one. He wants to die in the war for his country. I want to be with him. And he knows that. He says the same thing to me too. And that he wants to marry me, and i'm perfect in his eyes. He's only had one serious relationship his whole life. And the girl cheated on him. He doesn't talk to her no more. He tells other girls he wants to be with them too. But i don't know if he's just joking with them? And is joking with me? I really love this guy. I want to be with him. I fell for his personality before his looks. We dated a long time ago when i was 11. But i left him because he was to old and i didn't want my family to find out, and freak out. I need to know if this guy is serious with me or if he's just making me fall for him when he doesn't even know it. Please help me, i need to know before i get hurt.