The first place I would look for a replacement would be RELATIVES, his side-your side-inlaws... Relatives wouldn't feel bad at all and would fill in because "it's family".
I wonder if this problem has come up on those sites like "weddings.com". Do you know what I'm talking about? They may have a really good solution, because you can't be the first who has been royally screwed by a so called friend.
http://www.weddingbasics.com/attendants/forum_help.asp
edited to add the link above. It discusses uneven wedding party.
IMO-If your entire wedding is really formal and by the book, it might appear something is "off". If you could come up with some ritual for the odd man out to perform it could look like it was planned that way.
edited again! You might appreciate this little article as well. Talks about your problem. Says uneven is perfectly OK!
http://www.weddinggazette.com/content/003994.shtml
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