I've gotten way better at this. I've bought nipple clamps, and a few sets of lingerie that are cupless. And asking doesn't hurt at all!

That's pretty much what I was going to say. Does he touch your nipples at other times? Say to him while he's doing it, "I love it when you touch me there", gives him more confidence to wander around a little more during other times. Totally express how you feel. Once you open up about it, and aren't as shy, you'll be AMAZED at what your relationship can be like!

Absolutely not greedy at all, in my opinion! I think men in general are actually happy when you communicate what you want because it means that they can do a better job of pleasing you. It just depends on how you ask -- I find that a good way to do so is "compliment" + "suggestion" + "compliment" + "emphasis" + (optional thank you or whatever). That may be laying it on a little thick, but it's probably the most cautious way to do it. For example:

It feels amazing when you focus on my clit + but I like it most when you focus on my nipples + and you're so good at it + that you make me just want it all the time. If you find this to be a little awkward for conversation, during sex it might play out like this:

Express enjoyment with clit play + suggest nipple play + express even MORE enjoyment with nipple play + suggest he do it more often + reward him with something that he really likes.

This is how I get my partner to learn what I like without him even realizing it.

Definitely not greedy to ask for it at all! Be direct!

No it isn't greedy it is part of the experience with your partner

Talk to him! I haven't met a (straight) guy alive that would turn down playing with boobs.