Ahh, you need to train him out of any reaction to other dogs - if his leave is strong then id use this - if its not do some big time indoor training with it: food on the floor for 10 mins - playing with his favorite toys any and all reactions get a leave (obviously said before hand). Ask him to sit or go down whilst your doing this. When your out on lead with him follow this to a leave when approaching other dogs, try typical doggie walks times (4:30-5:30) so there should be a number. Id use lead corrections - with basic commands - leave, as your walking past. Any looks or tugs towards gets a command and then correction good boys when he walks straight.
This should be a routine for a good 2 weeks everyday after that see if you can trust him off lead with a leave, it sounds like the early stages really, at least its not developed into a full attck without reason.