1. I think he likes you but doesn’t want to admit it, or he is unsure of whether he wants to date you or not. Maybe he decided he wanted to stay single through college and now all of a sudden, you come along and his feelings are going crazy and he doesn’t know what to do. I would suggest talking to him, but I don’t think he’s going to talk to you about it until he wants to. So just be there for him, and wait for him to come to you.
2. The only way to know if he liked you in the past is, honestly, to ask. There’s no way to know for sure. Even if you do ask, you have to know that he could be lying about it. So there really is no fool-proof way. As for your second question, it just means that he sees you a little differently, I believe. It could be a good thing, it could be a bad thing. It’s impossible to tell from just one look.
3. There’s nothing you can say to make sure someone doesn’t stop loving you. It’s something that’s beyond your control. All you can do is hold on, and hope for the best.