Sport, you really need to train the dog to know "whoa". Whoa does not allow for fidgety or slipping, and if the dog breaks whoa you need to enforce it, even of you need to keep the dog ona lead or at hands length.

The good thing is you have a real young dog, who just doesn't understand yet what "whoa" means. Take the time for the dog to learn "whoa" on a lead, where you can reinforce the correct behavior. One thing you can do if the dog gets it in a quiet and controlled setting, but breaks as soon as anything distracts it is to work on smaller distractions in training, and work on the dog breaking, like tossing a ball or a treat when it is at "whoa". When the dog lunges, physically place them back at the starting point, until you release the dog.

Just curious, how have you done with your basic obedience training? Does your dog instantly "sit" or "stay" on command? Personally, I always trained "stay" with my GSP's first when they were pups, and when they would stay locked until released I then moved on to "whoa" in the field training. A gooddog can make the leap from "stay" to "whoa" in a training session or 2.