Depending on what you are using for a soundboard, some have a compressor that has an adjustable frequency trigger. If the noises you are talking about are close in frequency you could setup an aggressive compressor around those frequencies so it would not be as jolting when it happens.

You could try listening back with him and pointing out the sounds you are trying to correct.

If that does not work, Petsmart sells these training dog collars that give a corrective shock