Aggressive new dog.

[SteveInNOLA]

I adopted a terrier mix that was found on the streets from a shelter 6 months ago. I brought my other dog with me to the shelter to let them meet and see if he was a good fit for our family. When the dogs met the terrier showed no aggression (the dogs still get along fine). For the first month or so, when I walked the dogs they didn't bark or react to other dogs aggressively. Over the last few months the terrier goes totally nuts barking, growling, flipping around on his leash every single time he sees any other dog. When I take him to the dog park and let him roam off the leash he usually gets along just fine. I have been using only humane training techniques, but no matter how many times I discipline the dog (I try to catch him as soon as he starts to go into attack mode) he won't leave other dogs alone.
About to pay a trainer $250.00, but am afraid the little guy won't ever calm down.

Help!

[boxtops]

There are doggy people on here who I hope can advise you - but this is a UK site, so although the advice may be the same, the costs etc might be different.