I haven't read what the others have said, so excuse me if there is a repeat...
I tend to think that most people have been raised to have to lie and be secretive in order to get their needs met. From an early age we have parents that are flying by the seat of their pants with parenting and often miss what their child needs. What the root of their needs are. children tend to go underground in order to find what they need from others or by themselves. I certainly grew up this way. It wasn't safe for me to express myself in certain ways and I learned, as a defense mechanism, that I had to stay safe and find what I needed elsewhere.... in order to do so I lied and kept quiet.
What happens later in life for most, I believe, is that they have had trust broken and have had to get their own needs met often enough that when and if they find themselves in a relationship that requires them to be open and honest, they have to re-learn that way of being. That is what I have had to do anyways. I find that there are still things I don't talk to others about, but largely I am completely open, more than most as a result of pushing myself and others to talk and accept who I am.
Perhaps this is the reality for your SO. Maybe he has some re-learning to do. He might also just not be that in tune with what is going on for himself and has agreed to boundaries that don't work for him. perhaps he wants more privacy and covets the times when he is in his own head and enjoying remembering his own experiences. Nothing wrong with that, but you have to agree to less information.
Maybe it's time to look at that boundary again and see if it needs adjusting and what you will both say to each other to make sure there is comfort around the new arrangement. I find playing out a possible scenario works wonders to help get the words together in order to make sure they are fluid when the moment arises that they are needed.
Its hard to say what is going on without knowing a bit more about specifics, but I hope that helped.
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