M.F.
Why not get him a book from her? That way he can save it forever and read it to her when she's born. There's a ton of appropriate books on the market like The Daddy Book, My Daddy and Me (about 12 different books by that title), I Love My Daddy, Daddy and Me.... I could type forever. Then, the card just has to be something simple like "I can't wait to meet you." or "I'm looking forward to reading together."
Congratulations and good luck!