You need to let your dog is not OK to growl at your son . I would try feeding your dog then tell him to sit and take the bowl away for a second so he will learn that he can't growl at a person . I did this with my last dog as I did not want him to be so protected of his food when my grandchild was around.
I agree with this 100%. All food, toys etc. belong to you at all times. You and every family member should never ever be growled at, snapped at or bitten....but this requires work to solve. Seems like the dog would never do this to you, but feels comfortable threatening your son. Likely this has worked for the dog in the past so it keeps doing it.
I guess it could be testosterone so it may taper off....but if this type of aggression occurs with food, maybe it's occurring in other subtle ways as well? And if so, this needs to change for a household to run smoothly.
I recommend group obedience training from a referred trainer from someone you trust - Jack should go with you to this training.
I like the idea of a food dispenser for now, but I really think the obedience course can help dooger see Jack as a boss equal to you.