Cesar, the Dog Whisperer, recommends you redirect his attention from whatever he's fixated on. You do this by tapping him on his hind leg closest to you and firmly tug on the leash away from the fixation, oh and making a sound that will help break his focus. I do the "tch" sound. My bigger dogs get fixated with other dogs in the neighborhood so I do that to keep her calm.
Good luck!
http://youtube.com/watch?v=kNA8JbnKU94