has he been well socialized growing up? if so, it may be just something about these two that triggered him, but even so you need to nip this in the bud.

What I heard mostly in your story was that when he did act aggressively, he didn't stop when you tried to get him to so you need the tools to make him obey you.

I am going thru this exact same thing right now with koda. He went after a puppy the other day at the dog park and wouldn't stop and I didn't know HOW to correct him as he had never done this before.

If no-one here has any names, go thru your local pet store or yellow pages and find a one-on-one behavioral trainer that will teach you the tools to correct him properly.

You own a dog that, if not handled correctly and strongly, could cause some real damage if this behavior is not nipped in the bud. Thank you for being responsible enough to recognize this behavior and doing something about it.