aggression issues

Well.. one of my biggest fears seems to be materializing before my very eyes. My sweet, loving, friendly, "wants to love everyone" doberman is showing signs of aggression. Towards dogs.

Im happy that its dogs and not people, but I'm still not terribly happy that its happened at all. I was pretty psyched that he didn't seem to have any lasting issues from his attack a month ago... but it would seem that I was wrong, and that the symptoms have simply taken awhile to appear.

Yesterday I took Lexx for a walk after work. He'd been alone for 8 hours, so he was a big ball of bouncy excitement. I had planned on walking around the 1km block to the park to let him have a run, then to continue walking and pick Kyle up at work ( about another 1.5 kms away).

As we turned the first corner, my neighbour's mini schnauzer came bounding off his lawn, electric fence collar on ( and apparently not working), owners nowhere in sight. This dog is known to be dog aggressive, he's barked and growled at Lexx before and once when we let them meet he went for Lexx in a very rude fashion. Needless to say i was NOT impressed and kept him far from lexx with my foot until he turned and ran home. At this point I thought Lexx was being a good boy. He was excited that a dog-friend had run over to say hi, but was in no way acting badly. I continued our walk with a whining, trilling, bouncing doberboy who, in the most accurate way I can describe it, was PROTESTING that he had not been allowed to play. Tirill Trill Trill, bounce bounce bounce, grumble grumble grumble.

As we turned the NEXT corner we came across another neighbour with her 9 month old pug. She asked if lexx was friendly ( which i said yes to) and asked if she could say hello ( which i also said yes to). She approached quietly and lexx wiggled his nubbin and said hello to her. THEN he realized that she had the pug with him and as he stared at it, he started a low growl whcih transformed into a snarl. I immediately pulled him away and made him sit in a firm voice said "NO we will NOT have THAT behaviour". I explained that the other dog had rushed him and apologized for his rudeness.

She didn't seem concerned in the least but I felt TERRIBLE. This isn't being a good ambassador for the breed at ALL.

Now, several things happened here that were my own mistake.

First off, I knew he was frazzled and shouldn't have allowed him to say hello at all.
Second the pug is an unaltered MALE and I should have recognized this and not allowed him to say hello.

but im so used to my dog getting along with everyone, all the time. After seeing that dog fight I'm petrified to allow him to run free with other dogs again. I'm nervous that something will happen again. I'm sure that he's sensing that and going more on the defensive.

after i returned home, I mentioned the incident to Kyle who told me that when he was visiting his mom in PA with Lexx last weekend, a dog had rushed up the street side while they were walking him and barked at them. Apparently Lexx got huge and just growled and showed his teeth and was "very unhappy with this dog's presence" as Kyle described it.

I don't so much have an issue with him being defensive against rushing/barking dogs while he's on leash, but I'm concerned about growly or aggressive behaviour when its in a controlled situation, like meeting another dog who is NOT barking/running at him.

not sure what to do now...