Hi Jo I agree with the others, 'dominance' is such an overused word and your dog doesn't sound dominant to me, rather he is fearful and trying to prevent other dogs from coming near him by putting on a show of bravado.

You have had some excellent advice already , can I just add a word about the lack of socialisation. Here is an article by one of our members about 'Shy and nervous dogs' which you may find helpful. It describes a method of desensitisation which can be used/adapted to help with a fear of strangers and other dogs.

http://www.dogsey.com/dog-articles.php?t=8050

One of my Beagles started to do the dominance/fear aggression thing.How i got him out of it was useing a citronella spray collar. I know they are not cheap but within two long walks around plenty of new dogs to practice on we never looked back.

But, spraying a fearful dog could make the problem worse.

What your doing is showing your fear and anxiety by shouting the dogs name or trying to get into the middle of two fighting dogs. What your doing with the Bleep or Spray on the collar is changeing what the dog is thinking. The collar dose not scare the crap out of the dog just breaks up the thinking.

I do not agree in any way shape or form with the above. Can I suggest you read The Culture Clash by jean Donaldson and follow the excellent advice given by previous people. Frightening a nervous dog is pointless and cruel.
I have a dog who was completely unsocialised. Yes, sticks, umbrellas, ladders, small children, mobile phone users were all strange and worrying to her. Two years down the line she is fairly normal. Take time to retrain your dog and you will have a happy friend for life. All short-cuts as with sprays, rattles, shocks, kicks (sorry `nudges`) do is to complicate the problem.

I would think the OP is looking for a solution that will work well before dogs start fighting,

That's right! I've had this mentioned before but I'm not a fan.
I have to say that I was of the belief for a few years that he was afraid, but had been convinced that it was dominance by other people.
I'd love to be able to trust him off lead (he's actually worse on lead with dogs as he can't escape but off lead if he goes at a person I can't get through to him as he is so fixated on what he's doing). He does have some friends, but new dogs don't stand a chance.

Sounds a lot like my Weimaraner and she is fearful NOT dominant, I have to agree that dominance is often very misunderstood, unless it was someone experienced in dog behaviour that told you this i'd take it with a pinch of salt.

Cant really add to what has been said only go prepared with tonnes of patience and try not to take him out of his comfort zone as that will only send you 10 steps back.

Oh and please don't use a spray collar at the moment as this is only likely to have an adverse effect if he is fearful and will enforce the fear.

Random has lived with a situation similar to yours, {though I believe maddy is far better these days,}

I take aggressive dogs on to work with, I have to say, over the years most of the dogs that have came to me have had aggression issues due to many reasons. A truely dominant dog rarely turns up,