I am personally, the type of person that has a tendency to become withdrawn and sullen. No major reason, just the type of person I am. Part of my own past, neglect, being raised an only child, on and on. I sometimes, also become quiet and withdrawn, when tired, or really thinking about what someone said. Instead of coming out, with my opinions, I may sit and ponder. I've noticed this about myself, more. Yet, when called out on it, *lightbulb moment*, oh yeah, I'm not withdrawing from the person, but more *oops*, guess it's time to share what's on my mind.
That said, being withdrawn may be your fellas way of just being. It's when a person, is unable, unwilling, to just open up in the moment, and admit to what's on their minds, due to not being in touch with what their opinion is, is where there arises concern. Experience tells me, there's a difference from being withdrawn/sullen/whathaveyou and the silent treatment(which I have encountered in my former marriage--those are no fun)
I get what your bf means about tossing out the 'I love you's. I am like that. More now, than ever before. I found, from my past, how meaningless those words appeared in light of the struggles I endured. Actions, do speak louder than words. I don't, personally, need to hear those words uttered, to feel secure in a secure, comfortable, respectful relationship. There's many other ways, even in a LDR, to show how I feel and vice versa.
LDR's are tough. I get the notion of 'we really don't know each other' being said to you, YET, to me, that sounds like a passive statement that masks what he's really feeling deep down inside. If he's having doubts, it's up to him to express them, in a less cop-out statement fashion. Needs to say what he means, exactly, what does he mean he doesn't 'know' you. Being friends for years, and during this one year experience, why wasn't he asking you who you are deep down, where do you come from, what are your opinions on life, what are your personal life philosophies, how have you handled strife, so on and so forth. By now, especially with the distance, he Should, technically know you from the inside out. And even the quirky habits that you have, as during a LDR relationship/holding pattern, you'd have been as forthcoming as possible, little things about how the toothpaste is handled, cabinets shut/open, toilet lid stuff and all those little things that can annoy the p*** out of people, or not annoy people. By now, you'd expect to understand spending habits, credit stuff on and on and on. That's the disclosure that should occur in an LDR when it's talked about becoming more serious, marriage included. ((which, with the LDR, you do have the luxury of knowing all these things and more, and not be so quick to mask it with intimate moments under the sheets))

Don't know, what to tell you, if he is getting cold feet. I just know, I wonder what you mean about having been through H*** together, already?