I confess I'm new to the forum so I'm not sure if this will help, but I have a dog who can be dog aggressive, she's extremely selective of which ones she likes and doesn't like with no consistancy. For me it's easier to assume she's going to be aggressive and keep her away from the situation completely. I never take her to the dog parks or beach (anywhere where there could be dogs off leash) and when I take her for a walk she's always on a leash and I never let her go near any dog that another person is walking (leashed or not). She may be fine with that particular dog, but I'm not willing to take the chance of causing damage to her or another dog. If it makes you feel better, I've also seen much worse, there's a dog in my other dogs training class that when he first started coming had to be muzzled and wear a pinch collar. He also had to be kept well back from the rest of the class. The owner said he adores her and her husband but won't even let people in the house. Since she's aware of it she usually muzzles him when people come over and she never approaches other dogs. He's gotten better in the past couple of months, she had friends staying over and he was fine, she also removed the muzzle in the last class and joins in with the rest of the group. She's aware she can't fully trust him so doesn't allow him to interact with other dogs beyond just standing and watching during social time, but he can now be around leashed ones without lunging. So I guess my long winded point is that since you know it's a problem keep him out of the situation and know you can't allow him to approach other dogs or other dogs to approach him, it only takes 1 second for him to do something that may have lasting problems. Since you said you've already tried training and it didn't work, I'd just always error on the caution side and assume he's going to be aggressive. It save both you and another owner a lot of problems.