Okay...I have three kids, ages 1, 7, and 9. My 7 year old would not DREAM of saying something like that to someone, and he wouldn't have said it at 6 years old, either.
I taught my children very early on (discussing things like this at age 3 and 4) that certain things were not nice, and that they hurt people's feelings. I absolutely would have not let that comment slide with a 6 year old, especially when he knew full well that you were not pregnant. He was just being a
and he knew it. Kids can have proper decorum and manners at age 5 and 6, for the most part. They can learn boundaries and discretion...and in my opinion, ADHD doesn't have anything to do with being rude. He would have been reprimanded, made to apologize, and gotten a priviledge taken away for sure.