Hi there.  To rule out any inappropriate behaviors that are being done to your dog, it's time to observe your son interacting with the dog, and then possibly have a talk with him.  If your son is making the dog nervous, or is trying to punish him for anything, then a serious look needs to be taken at that.  Teasing the dog, taking valuable things away from him, towering over him and grabbing at him, and trying to "be dominant!" to the dog are all big no-nos - especially in a situation where the dog is warning your son that things are not cool.

I am not trying to lay all of the blame on your son, but it's vital that any questions about his behavior towards the dog are cleared up completely.  If you have access to a positive training class (Petsmart has good ones, and trainers can also be found locally through www.ccpdt.com)   then it would be great for your son to learn about positive dog training there.  Good luck!