It used to really annoy me when we were dating and a couple of times in the car I would just respond for him, like this:
BS: "What do you think, [BS then-boyfriend]?
BSH: silence, no reaction
BS: "Well, BS, I think that that is a very good idea. Did I mention I love you?"
BS: "I love you too, why don't we go to the ocean tomorrow."
BS: "Great idea, I'll pick you up"
And on and on until I ran out of conversation ideas.
He didn't like it at all when I did that, and we ended up having a discussion about it. He responds more often now, even if it is just "okay". Sometimes I have to ask if "okay" means he agrees with me or if "okay" means he heard me but that's it. Not to say that he never does it anymore, he still does it a lot, but a lot less than he used to.