Really I have to agree with Bailiff to a certain extent - in quit walking your dog in areas where he attracks these dogs. Whole Dog Journal has a number of articles about working dogs under threshold for these situations. Of course that means you can't expose yourself to places where you encounter loose dogs because they don't understand to stay away. (out of the threshold). This is where I would think you will need to go for right now (work underthreshold) to build your dog's confidence.
A request to other members of the board - can we please keep chastisments to PMs rather than sidetracking a thread?