If I may offer a perspective from the other side of the gender coin:
He may have a guilt complex, or the answer could be simpler. There's something that he has niggling at the bottom of his proverbial chest and he doesn't feel like he can get it off. Maybe there's something happening in the relationship, be it physical or emotional and he fears that telling you will hurt you and/or the relationship deeply.
He may also be listening to that very common societal and male-imbued instinct that most women do not like to share, no matter how much they say that they do. I have been in and out of the poly lifestyle for a couple of years, and perhaps it's just my partners.. but despite being open, they all possess a certain level of jealousy. So too have I. It's human nature to be at least a little bit protective of what you see as 'yours'. Even if that person is with someone else, and you want them to be happy, it takes time to conquer that. Whether that's him, you, both, or neither, only you know.
Finally.. sometimes we simply know that something dangled in the face of our loved one only makes them angry, or brings about hurt feelings. In those instances, it can feel easier to just avoid mentioning them to keep the peace and spare those feelings. Right or wrong, that is what it is.
I wish you luck in figuring it out!