Once Bitten, Twice Careful!

My 8 year old son was attacked last week by my neighbor’s German Shepard dog. I don’t know what provoked the attack because my son said it just came at him and started biting. He was in the backyard with my neighbor who is a friend and he knows the dog well and the dog knows him. If the owner hadn’t been only a few steps away I am sure the dog could have killed him. As it was, he suffered serious bites to the side of his face and to his hands and had to spend 2 days in the hospital while they cleaned and dressed his wounds. He is home now but he will very likely have scarring on the side of his face for life. He was lucky I guess that the front of his face was not very affected so he will still be a handsome person I think. The dog has been put down and my neighbor is very distraught. I believe it was an accident and so I am not looking to pursue anything against her. My son has been having nightmares and seems very upset and scared still about what has happened. He will not go to sleep in his own bed and he is just not his usual happy go lucky self. His best friend has a golden retriever who is the sweetest dog you’d ever meet but my son does not want to go over to his friend’s house to play anymore. I am worried that this might have lasting psychological implications. Does he need to get some therapy as a preemptive against problems or should I just keep my eye on him to see if he continues to have problems. What should I be looking for as warning signs that he is struggling emotional and mentally from the attack?