If he didn't find you somewhat attractive, he wouldn't be with you. you're assuming that he's driven by boobs which is more an insult to him.
If your bf didn't have any hands, would you look at other people who had hands? Or if he was bald, would you stare at men with a head of hair?
1) It's natural and sometimes unavoidable to look, especially if cleavage is very noticeable.
2) It doesn't mean that he finds you unattractive, it's just something different that he doesn't see often. Kindof like, if you saw someone with rainbow hair or with a facial scar - you'd look. It's new and different.
You could mention to him that you've caught him staring at other women's boobs ad that it's making you slightly uncomfortable or concerned and see where it goes from there.