First of all the dominance thing is total crap and has been debunked. Did the rescue assess the dog with children? Dog is in a new place with all new experiences, you know he is safe and loved, he doesn't. Get back in touch with the rescue and tell them what is happening ask if they have a behaviourist who can help. You may decide to return the dog, that is up to you. If you decide to keep him theme give him loads of space, places to hide and be patient, se a baby gate or similar to keep him and your son apart for th moment till he settles. There is a great rescue and advice site called Dogpages ((google for it) the forum there has loads go good advice and experience. Please do not be tempted to try and enforce your will on the dog, the growling is saying he us not happy, it is possible to suppress that then you have. Dog whose only way of communicating fear and unhappiness is with a snap.