Boy won't stop looking at my boobs?

Yesterday I was at my local country club pool with my best friend, her sister, this boy from my school, and his cousin. I was wearing a bandeau bathing suit and the boy wouldn't stop staring at my boobs. Maybe it was because my best friend is only a 34A and I'm a 36B...but I'm not that big. Why was he staring at my boobs and how can I prevent this in the future? By the way I only have bikini and bandeau bathing suits for the summer. Thanks!