First, he probably looks at yours when you are not looking. Guys don't want to get caught looking at any girl's cleavage because many then label them perverted. Tell him about your feelings and that it would make you feel better to know that your efforts to get his attention are being rewarded. Once he knows that you will not be bothered by that, he will probably pay closer attention.
Second, I am guessing he has seen a lot of them, so it is easy to not pay as much attention to something you are completely familiar with. But even a guy that has been married for years, will still look at his wife's cleavage some of the time.
Lastly, my wife realized that men look. She often told others that she expected that I would look at others, and that if I ever stopped looking then I would be too old and decrepit for her. :-)
Just for a change of scenery basically. Don't take that as a sign that he intends to cheat on you. Remember that guys are visual creatures, more so than women. Just because he looks at other women doesn't mean that he thinks any less of you. He's still with you, so that's proof that he loves you. I've worn low cut tops and guys that i know have gfs always look at my boobs. In fact sometimes you dont even need to wear revealing clothes to get guys checking you out. I used to work in a warehouse and some guys in there used to ogle me on a regular basis. Don't you take a peek at other guys six packs, pecks or arses. He probably still does look at yours when your not looking.
Because he's already seen yours so many times (IMHO he probably does take a peek once in a while but you don't notice) while the other girls are new to him. This doesn't have to mean he wants to cheat or doesn't love you.
Most likely here, dear, he is so Used to Seeing what is Part of Your Package, he just takes them For... Granted. However, if this SO is Looking and Lurking at other Chick's boobs, maybe it's High Time To... Hide them so he doesn't Smile.. Meaning, Cut him off for Awhile. Good luck. xx