Hello.  Would welcome advise regarding my 2 year old sheltie.  He got jumped on and attached at 12 weeks old by a far bigger dog.  This terrified him naturally but now he's older (almost two) he seems to thinks that barking and making himself look fearce is the best defence.  I have five other dogs who he gets on well with.  I've been taking him to a few companion dogs show (not to compete just for the socalisation) and he is getting better.  Luckily I live in the country so don't meet many people on walks.   He is also very territorial if we have visitors.  He's fine if they just sit down, but if they pick something up or get up to leave then he headbutts them!  No teeth involved but it's scarey for them. 

I think you really need to get advice from somebody who can watch him 'in action' as it's very difficult to advise without being able to see the dogs body language.  you may find that taking him to places such as shows while possibly 'flooding' him with stimuli, could also be facing him with so many decisions he can't learn a correct response.  I'd be more tempted to take him to the corner of a park away from other dogs and find out his 'tolerance' distances.  You can then work on rewarding him for NOT responding to dogs outside this area, then slowly, very slowly decrease the distance between you and the other dogs (perhaps a friend with an easy going dog to start with, or even a friend with one of your own dogs) and reward him for carrying on the correct behaviour.