It seems that he is falling into the whole notion that its okay and normal to lie and cheat rather than be open about what we do. It seems that to a lot of people it is better to have the facade of monogamy because that is respected, even if there is cheating going on. Cheating has become normal and even expected in todays society it seems. In some ways it is indicative of the numbness to feelings that our culture now has... numb until we are blindsided by them in the case of being the one cheated on. The one cheating is sort of treated like "meh, whatever, thats just what happens" and carries on believing they have done no wrong.
It might be possible to smack him up side the head with what it feels like if you create a scenario for him about what he would feel like. If I were you I would create for him a scenario that you might be interested in someone and intend to ask them on a date, even sleep with them at some point. Ask him how that would make him feel. Then ask him how he would feel if you didn't bother to tell him. Chances are it will evoke some kind of empathy from him.
He might say that he would rather not know about your pursuits; don't ask don't tell (DADT) kind of thing. If he is okay with that in you then fine, but you are not okay with that in him and those boundaries need to be discussed and an agreement reached. Request how you would like it to go next time and ask him how he would feel about changing his routine to consider your needs in the whole thing. How would he feel telling you about his interests and how much does he want to say? How much do you want to hear? All negotiable and valid questions I think.
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