You can put him at ease by returning a warm smile, then the next time he sees you he will be more assured in his smile.
Do you come across her on your way to college? It's not just a stereotype, it's science: Eye contact is one of the most consistent signs of attraction scientists have found.
He makes excuses instead of attending things you invite him to.
As Sassoon says, these changes can point to a deeper level of closeness and trust.