There really aren't any good nicknames for Todd as it can't really be shortened. It's in cases like these, when people feel the need for a nickname, that the name is made longer.
There are far too many men out there trying not to lash out when referred to as "The Toddster," "Todd-o," or "Toddzilla." Just as bad, are the patronizing nicknames of "Toddy," "Todders," or "Toddle."
If you're looking for a lasting nickname, go with something unrelated to the name. If you're looking to make a fun little game of it, try calling him something different each time. Make up some strange name that Todd could be short for or invent a strange middle name to accompany the short given name.
"Nice tie there, Toddbert."
"Excuse me, Toddifer."
If you have to think hard on it though, forget it. Nicknames are meant to be natural, never forced.
Todd Nathanson goes by Todd In The Shadows.
Todd Sinkewich goes by Todd Sink.
Todd Shaeffer goes by Oh my Todd!.
Deborah Todd goes by Deb Todd.
Todd Stansell goes by Todd Zane.
David Todd Ocvirk goes by Todd.
Todd Sklar goes by The Freewheelin' Todd Sklar.
William Todd Stinson goes by Todd.
Todd Rundgren goes by Runt.
Todd Pratt goes by Tank.
Todd Neilson goes by TodZot.
Todd Niesen goes by "The Captain".