"Mom, when I'm big like you and Dad, will I still get to be with you?"
"Seth, when you're an adult, you'll have your own family that you'll live with."
"But Mom! I want to stay with you and Dad!"
"You don't want to grow up and live with your own family?"
"Well, yes...I want to grow up but I want to stay with you and Dad."
"Okay Seth, that sounds good to me."
(Rather than argue, I decided to just agree with him. I'll bring this up when he's a teenager and anxious to move out and be on his own.)