Thank you for the really quick response and advice! We will give it a go.
The growling is specifically when my son tries to pick her up when she is seemingly at a point where she doesn't want that. For example, this morning, when she needed to be moved temporarily from a favoured spot. Most times when my son picks her up, she has no issue. She has never growled at any other family members regardless of when or from where she's been picked up, only with my son.
Is it possible some form of hierarchy of the family structure has become established in the puppy's mind? From my various searches online, some of the suggestions I've seen ask that the person at who the dog growls becomes the main feeder and walker, thus apparently establishing them at a higher level in the family hierarchy than the dog.