This is a difficult question because it depends heavily on the kind of person your boyfriend is, and the type of relationship you have. 

In my experience, the best way to handle these situations is to first evaluate how the other person would feel about you sharing this type of information. Almost everyone is experienced enough to realize that their partner will at times have physical attractions to other people, but many are people are happier to allow that possibility to exist in the theoretical realm rather than face it directly. 

Here, you can start out with 

> Hey boyfriend, often times you mention when someone that you see or know is attractive. How would you feel if I was open with you about the people I know that I find attractive?

The most important thing after he lets you know how he feels is to respect his feelings. If he tells you that he does not want to hear about these things, it is probably best to let it go. Again, any rational person will know that their partner will have attractions to people at various times so it is not dishonest or "cheating" to have these feelings **even if you don't explicitly discuss them**. 

In your question, you seem to have framed your relationship in such a way that you are both capable of these conversations, so lets assume that is the case. 

If frame the conversation the way I have suggested above, it is unlikely your boyfriend will raise issues of your faithfulness because you have already drawn comparisons to his own behavior. In this case, you can simply say something like 

> I'm glad I can be open with you about this because I love you, and I really value the honesty in our relationship. I happen to find Greg at work attractive. 

Phrasing things this way almost leaves Greg as something of an "afterthought", as indeed your attraction to him really is an afterthought compared to your relationship. 

Pay attention to your boyfriends reaction when you tell him this. Leave it up to him to continue the conversation as he sees fit. He will likely have some questions, and you can clarify these - but pay attention. If he looks clearly uncomfortable it is probably best to not unload a bunch of information detailing your crush right away. If this is the first time you have discussed something like this, it may take him some time to process this attraction and come to terms with it. 

All in all, start out gently and let your boyfriend continue the pace of the conversation as he sees fit. You can always come back to it later if you feel like important details were left out, and if he is still comfortable continuing the conversation. 

Hope this helps!