Find a time when you can talk with him alone (or ask him specifically if you can have a few minutes of his time alone).
First, say you don't know whether he's aware or not but you'll be moving away soon to (name the city, state).
Second, tell him it's important that you share with him that you've always thought he was (smart/special/kind/funny -- fill in the relevant adjectives here) and tell him what you like about him. This isn't awkward. People love to hear positives about themselves!
Third, tell him that although you're moving away, you don't want to lose contact. Give him your social media, phone, and other key personal contact information. Request his too. Take a selfie together if he's willing. That might sound awkward, but you'll be gone soon and you'll want it. You can also use it as an excuse to initiate contact with him via text before you leave.