What does it mean if a shy guy calls you darlin?

I texted happy birthday to this guy I've got a crush on. He's quite shy. I don't know him in real life but we've chatted over social networking websites for years. We tease each other quite often but he is reluctant to meet me in real life because he said it would be awkward. He replied saying 'Thanks darlin xx' so I was shocked. He doesn't put on Facebook that it was his birthday like most people do so maybe he didn't expect me to know it was his birthday and was surprised?
What does it mean if a shy guy calls you darlin?
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