Dixieland Dancer mostly covered the dog-human slant on dominance but I would like to add one thing to it. It is mostly the affection, the hugs, kisses and cuddles that are given as if to another human that trigger such a response in a dog that feels it is superior to the human in question.
Take a few minutes to think of how most humans, especially little ones, show affection to the dog. They reach around the dog's neck, apply pressure and encircle the neck area.
A hug.
A very strong signal to a dog that another member of the pack is trying to gain, or maintain, dominance is the simple act of resting or positioning the head, leg or lower jaw over the neck area.
We see a sweet and gentle cuddle from a little person to their loved pet.... a dog that is not secure in its position sees a small, weak, unsocial ( in dog language ) competitor making moves it can't back up. The dog gives the usual signals that any other dog would understand to mean, "Back off matey, you can't win and if you push me I'll prove that you can't!"
A small child will, obviously, not understand what the dog thinks he has made perfectly clear. Thus the dog is forced to make his signals stronger and less likely to be challenged.
If steps are not taken to rectify the staus quo right now, assuming that the dog has not got a medical problem, then the situation can only go one way.
This is not the fault of the dog. The dog has done everything it can to say very plainly that it knows it is in the stronger position and is willing and able to prove it. It is essential that you get a behaviourist to help before the dog is forced into action that will end it's life and may have life long repercussions for your little boy.
If you think I can help please email me.