Well, actions speak louder than words, sweetie. If he's no longer calling to make plans then he's not as interested as he was before. If he hasn't called in two weeks, I'd say that he's taken a powder.
Why...is a completely different question.
If you really want to know why or what happened, call him and ask him...in a very calm manner. If he can't really respond, I'd personally just let him off the hook without a temper tantrum or anything. Although I would probably let him know that you didn't appreciate the way that he just faded away and that it would have been nice if he'd just told you that he wasn't interested in pursuing the relationship. Then let it go...for your own sake.
Sometimes we lose interest in someone for various reasons, and it's not something we can help. How we deal with that depends on the integrity of the individual.
Too bad that he didn't have the class to tell you that he didn't want to see you any more. He apparently just decided to fade away instead of behaving like a mature man.
I'm sorry, sweetie.