A lot can change in the time between sixth grade and high school. You do mention that he already knows you like him, so I bet you've made that pretty clear. What you want to know is how he feels about you.
First, the practical. You've moved away and are in high school so unless you're in the next city over, your chances of an in-person relationship with dates, hand-holding, afternoons out with mutual friends, etc. is pretty slim. Even for adults in a mutually committed relationship, long distance dating over an extended period often becomes too burdensome. However, you can still have a "virtual" special attraction for a partner and make it work with social media. Don't expect it to be a full stand-in for a boyfriend that is with you in person, however. I urge you to try to look around at your in-person options, too. Keep yourself open to those possibilities instead of being 100% stuck on your long-time crush. In-person dating gives young people a chance to practice important relationship and social skills.
Second, decide whether you'd rather 1) know the truth about how he feels and risk rejection or 2) continue to flirt with him and anxiously wonder. If you want the truth, then chat with him through FaceTime, text, a call, or instant message. Tell him in advance you've had something weighing on your mind and need a few minutes to chat with him. After some small talk, ask how long you've known one another and go over a little of your history, encouraging his recall and participation in the conversation.
For example: "How long have we known one another, John? ... We've been together through x and x and x. I wish I had never moved away, but I'm glad we're still in touch. ... You know I think you're pretty special, don't you? ... I was wondering how you felt about me?"
If he doesn't like you back, he will probably want to end the FaceTime or instant message quickly, and he'll use language such as you're a "cool friend," a "nice person," or "sweet girl." If you want to be super clear about his feelings, you can say, "So just to clarify, you like me as just a friend, and you could never think of me as more?"
That should do the trick for you. Again, I encourage you to start looking around you, too. Go out with mixed groups of male and female friends on friend dates to the movies, putt-putt, to the mall, etc. You never know.