It may depend on how long you have been going out. If it has not been very long he might be worried that if he went straight for the breasts he's worried he may look perv**ted and that you might think he's only in it for sex. No offense to you of course, but that is the reason then he's probably quite sweet.
If you have been going out for a while however then personally I have no idea why. He just might not be a breast guy?
If you really want attention to your breats then take the lead sometime. Roll on top of him or handcuff him to the bed (which ever sounds best) without being to pushy move your breasts slowly closer to his face and tell him if he wants to be released he'd better carress/lick/suck/stroke/kiss them. If he doesn't seem to enjoy it stop as soon as possible. If he does continue until the enjoyment seems to cease a little too much. Then you'll know (I think).
maybe they small and all he wants is pussy ... =]what are some fun like sex games me and my boyfriend can play over texting