First off, i like both of your replies very much Canis-Lupess. You pretty much spoke what was running around in my head. From that i have almost no advice left to give, but only comments.
I'm wondering if it's not truly aggression, but dominance?
When we first acquired Koli she was mostly mis-diagnosed. She did have a true bit of fear aggression, but it was mostly just her having a strong alpha personality mixed with a huge prey and herding drive. She was like an un-trained spastic border collie who had just escaped a small crate. The methods the trainer was giving me was actually making her become aggressive. Again... the see another dog, get punished line of thought. (Though this was with the Halti... and it was just her pulling on lead.) I knew something was wrong. Still i was at my wits end and really thought i was going to have to have her put down. Someone stepped up though and offered socialization consultations. They have done wonders!!! It's almost like having a whole different dog. Her fear aggression melted away. Slowly she's learning how to not be too domineering. She's learned how to play... nice? Nicer than the games she was playing.
Anyhow... from that, i suggest finding people that are willing to socialize their dogs with yours on a one-on-one basis. Or two with yours. Take things one step at a time. You are doing a good job already by learning his signs. Desensitize him and work on behavior. Praise him for when he thinks about the wrong behavior but backs down from it. Praise him whenever he's showing desired behavior around other dogs. It will be well worth the time spent. (and you have to have a "play-date"/"socialization session" at least once a week.) I'd say start off though with a certified behaviorist who have dogs specific for this purpose. Yes, they do exist out there. (If you can't find one or can't afford it, anyone with a mild tempered dog and they themselves are will work.)
Oh, and i agree heavily... too often humans get involved when they shouldn't. With dogs that aren't truly aggressive sometimes it's best to let them work it out. (It's a lot easier to remain calm when both humans are on the same page. ...which you'll have to make sure of that when looking for another dog for the socialization.) ...Oh, and having the leashes on both dogs can be a bad thing... Tangled leashes can lead to real fights and real bad injuries.
If you have the time i highly suggest reading "The other end of the leash" (I can't remember her name, but i know the author lives in WI.)
Originally Posted by Canis-Lupess
Let nature guide your actions and you will never have to worry if you did the right thing. ~ crow_noir
The pet world excels where the human world is lacking; sterilization and adoption. ~ crow_noir
Please, if your dog is arthritic look into getting it Elk Velvet Antler. Look up my posts on it, PM me, or look it up on a search engine; but please if you love your dog and want it to live many more years consider this option. I've seen so many posts on here about dogs needlessly suffering. I can't make a new post about EVA every time so this plea is going here. EVA also helps with other ailments such as anemia.