I would say that if he "stealth walks" he is not complying with the Whoa command.
Important thing is to collar-condition the dog. Many dog owners want to punish the dog for non-complaince (P+) with an e-collar, which is not a good way to work things. Conditioning with the collar includes ensuring that the dog understands what turns-off the collar (P-) as well as what turns it on. Collar Conditioning is done as part of a regular training regimen, so without knowing how you train, I cannot tell if an e-collar will suit your needs.
I collar-condition my dogs over a series of months in daily (4 - 5 days a week) training sessions, starting with putting the collar on, without even intending to use it, during leash training (a month, or so). I then go through ALL of the commands with a P- on the leash: I apply light stimulation (dogs ears perk, or it looks sideways when stimulation is applied, but the dog DOES NOT VOCALIZE), then give the command, and release when they comply (supplimented by a treat, or R+ - especially early in the process). I transition through this, applying stimulation later, and later with regards to the command, until it is actually a P+ for non-compliance (I don't apply stimulation if they are complying immediately). Once through that on all commands, I turn up the intensity a notch (depending on the dog) for application of punishment (P+), but find I seldom need it - except for chasing, and other distractions.
If you jump to P+ with a collar without conditioning, you can expect some major regression in your training, and you will have to re-build trust between your dog and you before you continue.
For more information, you might check out www.tritronics.com/content/wp-content/uploads/2010/07/manuals/basic_training.pdf (I use a tritronics when training), or there are a number of other books, and articles on the topic.