If you go out to dinner, go where they have paper tablecloths where people often write on it -- play hangman on the paper tablecloth and you do the phrase "another baby in December" or "we're pregnant."
Maybe you could paint ourself a shirt that says "Due In December." Of course, you'd have to wear a jacket over it, which isn't quite practical for Summer in Texas.
You could always get a shirt for your son that says "big brother" and say "look at the cute clothes I got Joey today." Go through a couple regular ones first, then the "big brother" shirt and see if your husband gets what you're trying to tell him.
At dinner, pull out an envelope and say "Oh yea - I almost forgot - This came for you in the mail today." Have it be addressed to him from "Mother Goose" or something like that. Inside, have a piece of paper that says "Thank you for your interest in a family. Your new child will be arriving in December 2010" or "Please allow 26-28 weeks for delivery." I bet if you typed it up real cute and actually had a stamp and mailed it to yourself it would be pretty unique.
Whatever you do, have fun with it and CONGRATULATIONS!