According to my dictionary, the word 'say', when used as it is in your text, is used to attract attention.
It's a shortened form of 'I say!', which indicates that the speaker wants someone to pay attention to what he is about to utter. Examples:
1. I say! What about a ride in that speedboat?
2. Say! You know what I've been thinking?
3. Say! Can I borrow your bike?
參考: Bloomsbury Concise English Dictionary. 2nd edition. A. & C. Black. London. 2005.