The only thing I can think of is that with 2 older siblings, the opposite of what you would think would happen is occurring. You would expect him to be able to converse better, but PERHAPS he isn't really given a chance. Maybe all of the talking is done for him, if you know what I mean? Mom, Dad and sibling all make the decisions for him, or don't really give him a chance to provide any input.
If he seems to be understanding everything else, I would just keep at it. Continue to ask him those open-ended questions, and try not to "give" him the answer. I would bet that one day, he is just going to start chattering away, when he realizes that he has some opinions too!