Our Jack Russell "Jake" has the same problem as your dog. He will obey all commands except when he sees something smaller and furrier than him - then look out! Our trainer recommened the prong collar until Jake could learn that he would not get away with this behaviour. The prong works so that when the dog starts to pull the pinch he feels is like when his mother dog would nip him to correct him (atleast that's how the trainer explained it).
It has worked very well. We have added the command " by me" whenever he starts to pull and the collar engages. He is slowly learning that this means he can look but not touch!