Determined to meet other dogs - Page 1

A bit of an odd situation, my male GSD mix is a social butterfly, and I have been working on his manners to have him not drag my to every dog he meets. The dragging part has come along wonderfully, and he is doing amazing, learnt real quick that pulling me to them didn't get the result he was after. The problem is coming at the point of the actual pass when we are side by side with the passing person and dog. We will be walking along, see a dog, I put him into a focused heel, and he walks along like a true gentleman giving me his complete attention. Then just at the point we are passing the other dog he will either speed up and jump across the front of me towards the other dog, or put on his brakes with me still moving forward and go behind me to the other dog. I have no idea which way he will go at any time. If it was always front or back, it would be an easy fix. He never makes it to the other dog because as soon as he breaks the heel I pop his leash (flat leather) and make him sit to stop the forward motion. Then place him back in heel and carry on. He is not doing this in an agressive manner, he is an over friendly dog and I am trying to teach him some boundaries where other dogs are concerned, basically no you don't get to greet every dog you see, and only when I say you can not when you want to. It really catches people off guard though and frightens them, I know it would catch me off guard too if I was in their shoes. Here you have this apparently well behaved 80lbs dog coming towards you in a focused heel not paying you any attention, and then all of a sudden when you are right beside him he jumps at you. no preparation clues for the other person and they are completely surprised by him, and rightfully so. Any suggestions on how I can fix this? Like I said if it was only front or back easy fix, prepare to block with leg and keep moving, i already have his attention, but somehow he has figured out my steps and calculates it based on where my feet are at the time.