I've had Charlie 10 weeks now, he's a 2 year old rescue dog (no history) who is settling in brilliant apart from out walking. I walk him every morning in the park for about 1 - 2 hours on lead, casey my other dog is off lead. We usually walk through with other dogs that he knows and accepts. If we meet a new dog at least half of them he will make an effort to go for them. I keep him on a tight lead so there is no chance of him getting to another dog. I let him on a long line when there are no dogs about as he tends to get a bit too excited when playing  then gets a bit aggressive. Is it just a question of time or does it sound like he is an agressive dog?  He's never shown any agression towards my other dog Casey or the dogs he walks through the park with. Would squirting him with water when he growls/barks at other dogs be worth a try? It works in the house if he is barking at people going past the window.

Is there any particular reason you don't let Charlie off lead as well?  It may be that what you perceive as agression is simply frustration at not being off lead and able to display normal doggy body language to play properly with his friends.  Also, by tightening the lead, you are sending signals to him that you are stressed by these encounters too, and many owners find that once they can learn to relax, their dog does too.....  If you're not sure about this - you might like to think about using a headcollar rather than a collar and lead as this will enable you to have more control over him.  True agression results in the other dog needing vet treatment, if this is the case, then you need professional help - see your vet!
I would never use an aversive like a water pistol as the dog can then learn to associate the thing it's barking at with bad things happening, which can increase their desire to drive the thing away :(

Charlie does wear a headcollar and I'm aware about tightening his lead when other dogs are approaching and don't do it, I keep one hand on the slack and don't let it go if I see another dog. I hope that makes sense. Charlie escaped from my bf's grip a couple of weeks ago and he just ran and ran, he wouldn't even come back when I shouted of my other dog to come back, he never looked back once, just kept riunning until someone kindly stepped on his lead till we got to him. I also wouldn't like to put any other dog in danger.
I feel awful about keeping him on a lead but would never forgive myself if he attacked another dog or got knocked over etc.
What would you suggest the best way to do this? Just let him off? Let him off muzzled?
I've never had a dog that I've had to keep on a lead for fear of aggression or running away so any advice would be appreciated.

When I got Dyaln at 14 months he'd never been out of the back garden (he even had fur on his paws!). I was told if I let him off, I'd never get him back. Anyway, after 3 weeks of walking on lead and doing wonderful recalls in the house etc I decided it was time to try it. I walked him up to a golf course (closed to players for the winter) which was completely fenced (ok so it was huge, but at least he couldn't get onto any roads!) and let him off... 3 hours later he got bored and came back! Admittedly he never really went out of sight for too long, but just refused to come close enough to go back on the lead (I resisted the urge to chase him!). Eventually I just sat down and we stared at each other for a good while until he decided that I wasn't so bad and he was actually ready for his breakfast. During his 3 hours of freedom he met countless other dogs and I think it did him the world of good (although probably not the recommended way of doing things!). He never showed any agression even though quite a few dogs were agressive to him, but he'd never fight back. Instead he would just look away and ignore them till they got bored and left him alone.

I had him neutered a few weeks later, and found this helped immensely both with recall and other dogs agression towards him. So, I guess what I'm trying to say is, unless you think he's actually going to be agressive rather than playful, it's worth putting some trust in him and letting him off in a secure area to run around and let off steam and interact with other dogs. Dylan had 14 months of steam to work off, but still came back eventually!! As long as you don't start making a game of trying to catch him, he will come back when he's ready. Gradually you can start working on calling him to you for a cuddle/treat, but then let him go again, perhaps giving him a game. Only put him back on his lead when he's come back to you and ready to go home (for his food!  :-) ) Once he's got complete trust in you he'll be more than happy to come. It takes a lot of time and patience and you obviously can't be in a hurry for anything else those days!! This may not be the advocated method, and it might not be appropriate for your circumstances, but it worked for me.

Hope that helped   :-)

Abby is 15 months and at the kevin stage she seems to have forgotten what recall is so she is on a lunge line and we spend  our walks retraining recall and teaching her to retreive(sp) I spoke to my dog trainer and she suggested doing this it makes me the most interesting thing in the park. It seems to be working slowly but surely.
Good Luck
Mary