We have all been hounded by elders I'm sure.
Here is what I would have said ( well, in a dream world where I actually could come up with it immediately, not thirty minutes after he left.).
"Thank you so much brother clueless, your concern for my wife and I and how we are handling the demands of a newborn is deeply touching. It was so kind of you to offer to babysit while my wife and I get some much needed sleep, and I am sure we will enjoy that casserole you brought. I know you totally understand that meetings and field service are not possible right now since we are so sleep deprived and exhausted that showers have become optional. And we will take up your kind offer to come over and help clean the house next Saturday. It's so nice to be part of Jehovah's loving organization and feel the loving concern of the elders! I am sure you won't mind showing yourself out. Don't let the doorknob hit ya where the good lord split ya!"