T.H.
I am not an expert with dogs or anything but my thought is that the puppy sees your son as mobile now and just wants to play. Puppies pounce on eachother and will run into each other trying to knock each other down. I am guessing he is just trying to be playful. Unfortunately your son doesn't understand this. You may need to keep them seperated when you cannot be right there, still ensuring you are giving them both attention. This way neither gets hurt or feels betrayed. BTW, your son will get even as he gets older, he will start pulling on the dog and may even try a taste test. And then the dog will feel picked on, funny how things come around.