We have no idea how long ago 3rd grade was. Last year? Last week? 20 years ago?
I highly doubt you are falling in love with him. It's far more likely to be a crush. These happen, they're a lot of drama but they do eventually go away. I am not saying that it is impossible that you love this guy, but you have no relationship with him other than being pals. Pals are nice, and they make us feel good about ourselves. But they usually aren't love.
As for the third-hand information about him supposedly loving you, forget it! Who knows how that communication broke down? That was a rumor, and nothing more. Rumors are, by definition, unreliable. There is also every possibility that whoever said this has/had an agenda.
Here's an idea. How about some direct communication? You know, not with your sister's boyfriend's dermatologist, but with the guy himself. You're chatting with him night and day, so you have ample opportunity. And he probably has no idea that you're entertaining the idea of waiting for anything.
So, next time you chat with him, say, "Ryan (or whatever his name is), I really like chatting with you and I'm glad you're in my life. Do you maybe want to get together some time? You know, not as just friends, but as a date."
Be prepared for him to say no, and he might. Will that make your friendship weird? It can, but it's currently already weird, if you're mooning over him and he's not reciprocating that.
Nothing ventured, nothing gained.