I didn't read all the answers but from experience I would suggest: does you boy wet himself when having a wee? Mine pees on his front legs and chest ... Other dogs smell it and try to dominate him. The more often I wash him (desodorizing) the less other dogs care for him.

Lol maybe your right! He always seems to want to play and never seem to incite the aggression and runs to me when the others become aggressive towards him so I always thought he was a bit of a wimp! But saying that I can't see any other reason for dogs to go for him so he must have some sort of superiority about him! How can I address this?

I had much the same problem with my boy when he was younger. Male dogs would cross the park to flatten him and pin him to the ground, he wasn't neutered either. I wish now I had taken much stricter measures to stop it as he became aggressive to every male dog (other than the ones that we knew well) that came across our path - attack being the best form of defence. It has taken a lot of time and effort but we are finally coming through it, however he does have to remain on a long lead as I cannot fully trust him not to attack if the vibes are wrong from another male dog. My advice would be to avoid, avoid, avoid until your dog is mature or you could risk having an aggression problem too. Only let him play with dogs that you know for a fact are bombproof. Protect him from aggressive dogs so that he comes to trust you will look after him and keep him on the lead around other males. I'm afraid there are far too many male dogs out there that are allowed to bully younger males and 'put them in their place' which seems to be what it's all about. I don't think it's 'superiority' at all, it's simply that he isn't as submissive as they'd like and therefore the other dogs go for him as that is what happened to them when they were younger - it's a vicious circle unfortunately. I'd be very rich if I had a pound every time another owner has said to me 'my dog's fine' and then they've pinned my dog. Of course it doesn't happen any more as he's five and huge but the damage has been done and we do have to watch him as I don't want him to perpetuate the problem with other young dogs.

You could try bringing something that he really likes on your walks, cooked chicken, boiled sausages etc.
When you see a dog walking in your direction, after he has seen it, hold a closed hand with the treat in front of your dogs nose to grab his attention. (my fella tries to eat it through my fingers.)
Keep his focus on you until the dog has passed, if he has maintained that, then give him the treat and praise him.
Watch the other dog to see if they react differently to a dog that's ignoring them. This little experiment will tell you if him watching the other dog is what's causing the aggression.
It will also train him that he doesn't have to meet every dog.

That's exactly how we sorted Mollie out, and meant when dogs now approach she looks to me quite naturally so breaks the stare even without a treat.

I would suggest that you do a search for Suzanne Clothier, who writes some excellent articles on training and behaviour. Her piece entitled "he just wants to say Hi" may be very relevant to your problem.

Hi Sometimes if you are nervy and worried about meeting other dogs your concerns will go straigt down the Lead,your dog will feel then that there is something to worry about and reacts accordingly.I think you should engage your dog,talk to him,tell him hes going walkies,make it into a big thing every time,and eventually you will both enjoy the walk and meeting other dogs,or if you have a friend with a dog ask to tag along.Gemini54

Hi thanks for all your advice! I have noticed that he is a very nervous dog and I am hoping that I'm not the cause. But this may be the reason that others have a go at him. He jumps at all loud noises and sometimes cowers especially around lorries. Not sure why he is like this as we have had him since he was a puppy. He also has lots of allergies which, I'm told, go side by side with nerves.
I am very vigilant when I take him out so as to avoid conflict but I don't know anyone who has a dog that I could walk with which is a real shame as he needs some doggy interaction.

If you could get here I can guarantee my two would walk nicely with yours, neither have a bad bone in their bodies real softies with all dogs. Where in London are you ? I am in Woking near Guildford.

Have you ever taken him to a training class ?