There are various types of electronic collar available and it is possible to hurt a dog with those that deliver a shock through probes touching the neck.
I do not feel the need to use one.
Teaching a dog to obey any command is more kindly achieved through the association of a reward with the action required.
You will have taught your dog to sit by your side on the verbal command using praise, and so sitting to whistle is taught the same way. When the dog is further away it is more difficult to deliver the reward at the right time, but not impossible.
Another alternative to a treat or praise is to use a retrieve as a reward every time the dog sits on the whistle, and this can be done when the dog is stopped at a distance.
Sit the dog down and walk away, turn, call it towards you, give one pip on the whistle to stop the dog and throw a dummy out to one side. Let the dog wait a few seconds and then let it fetch the dummy.
The dog will soon learn the whistle is the signal for something enjoyable and, when it is reacting well, the distances can gradually be increased.
Be careful not to let it retrieve every dummy in this way otherwise unsteadiness may be induced.