When my husband and I are watching tv together, usually Netflix dramas of some sort, it is a hard rule that if we want to discuss a character, a plot device, or want to puzzle out who we think is the murderer, we have to stop the program and discuss it.
We talk a lot. Usually current events but also philosophical questions about God and man, or history (he is a history buff) or sports. He is a fount of knowledge on many many topics. So we have a great time. We don’t talk about our “relationship” unless we have a conflict to work out. We can be silent too but it’s not really our style. We are both opinionated and passionate. Our son is the complete opposite. He is stone cold quiet most of the time. I haven’t a clue what is in his mind unless he cares to share it with me which he will occasionally. I imagine that he is different when he’s with his buddies.
Your son is a relationship voyeur. He is absorbing how you two do relationships. Not a bad thing just how one is put together in the noggin.