Here she is at 6 months...eyes on the Orbee ball, but holding the whoa.

Just a word of caution about the stuffed animals.  You might want to put a stop to it now as your pup thinks those toys belong to him.  At some point he will not just go get them, but unstuff them too.  GSPs love to unstuff stuffed animals.  I am only warning you as you said these were your kids toys and there might be some upset kids one day soon.  Substiture your kids toys and toy box with one for your pup.  You can even get stuffed toys for dogs, although he might have a hard time determining which are his and which are the kids.

Stopping the whoa dead in the tracks requires the more intricate setups I was hinting to in my first reply. It may involve the use of the whoa table, check cord contraptions to hold the dog in place from afar, etc. 

www.versatiledogs.com/ask/point31.html 

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Larry Mueller uses a barrel instead of a table, which really forces the dog to hold stockstill other wise he strangles himself...I doubt it is as cruel as it sounds (I like Mueller's general training approach), just very old school and not everyone may agree with that method.

There are many books, DVDs, websites out there, with info on how to teach it.

I disagree with the comment about not correcting the pup if it does not comply.. If your going to intiate any training and command it has to be carried through otherwise don't start it at all. It'll only lead to problems.. Correction is a simple pick and place, no comments or punishment is required just a re-affirmation of the verbal "whoa" command once the pups feet hit the ground.. Whoa training requires benchwork, the dog needs to be off the ground to feel insecure and focus on what they're doing, thus benchwork. Once the dog understands the command you can move to the floor/ground. The Larry Muehler barrel routine works well. The best method I've found which is similar was a harness and pulley. Hook a pulley in the garage rafter, in a tree etc. Hook the rope to a body harness on the dog. When the dog breaks, you or a helper then hoist all 4 of the dogs feet off the ground. The beauty of this is that you can do it almost anywhere.. I got lucky, I only had to do this exercise twice, she'll now stop on a dime even at 60 yards away.. 

Well what i meant by not correcting the dog I could of better said don't punish the dog.  Reality is most 12 week old dogs won't whoa like we would like.  Correcting the behavior to the expectation i would think is fine, but to instill some sort of harsh discipline would just frustrate the dog and prob create other problems.

The whoa barrel and body harness both work good but I don't think they are needed for the everyday Joe who spends time with the dog and incorporates the whoa into other routines.

I know a guy who got the majority of his Whoa training done with his screen padio door.  Before he would let the dog go outside to play....just as it would bolt to run out he would close the door in front of it (dog would at first run in to it) and say "who".   He said he did that  5-10 times a day before he would let him out to pee or play and before you know it the foundation for Whoa was done.

Its a fun thing to work on but like most other restraint to natural drive tasks it takes a bit to have it 100%.   Just when you think you have the dog Whoa broke, toss a living big old fat cackl'n rooster in his face....many will bolt after it!