Just so agressive

[Karen]

That's just wrong - the water-filled balloon - I've never heard of that, and it just sounds like a bad idea. What you CAN try is a spray bottle, and spritz him with water, to distract him. But a better idea is as soon as he starts to react to another dog, tug the leash a bit and get him to sit, then do "down" or "paw" or whatever other commands he knows, and only when he is calm again let him approach the other dog.

[Freedom]

What Karen and Moon & Bean said.

Be sure to always carry treats with you. I take a baggie with cut up bits of hot dog. (Did you know a hot do can be cut into 67 pieces for training? I've never made more than 60!)

You make sure the dog focuses on you, treat, get the dog to sit, treat. Treat liberally at first, to re-enforce the proper behavior. You can practice the "look at me" at home, even when you are alone.

Never heard of water balloons for training! Not very practical!

[akitrix]

Water balloon downright dumb & impractical, never throw ANYTHING at your dog, it teaches nothing good. If that's the only advice, then you need to find a more knowledgable trainer. Dog Star Daily is an excellent blog site staffed by some of the best animal behaviourists and trainers. Lots of good advice and insight.