first post on this board

Hello ladies - I have a question for the blended family board.  I usually post on June 2013 as I am due in about 5 weeks.

I also have an 8 year old step son. We have him every other weekend and extended time during the summer.  I would say that the arrangement / situation is pretty average.  Most of the time there are no issues and birth mom and my husband get along.  Occasionally there is a hiccup as well though.  (she's is pretty good about manipulating to get what she wants, and my husband is pretty quick to react) At birth mom's house, he also has a step dad and a slightly older step brother (who I think is there 50/50).   At our house he is the only child, and in fact only grand child, which we spend a significant amount of time with my in-laws.

OK so that is the background - here is my question.

Last weekend when my step son was with us he spent time with his grandparents without us.  When grandma brought him back, she shared that they were discussing the baby on the way because they were watching something on the TV that had a baby on it. My step son said to her, "Yeah it's Elizabeth's fault." My MIL said that she just kind of let it drop due to not knowing what to say to that. She wasn't sure if he was upset about the new baby, or if it was a comment that implied he needed a birds and bees talk (like that his daddy had a part in this baby too!). She left it up to us to approach it.

Anyone have any advice about this? This is the only negative comment I have heard him say. Most of the time he appears indifferent about baby talk. I think he's not quite grasping the concept yet as he has never really been around a new baby. We will only have one or two weekends left with him before this baby gets here.

TIA!