I doubt this is a phase , especially since "He was a very shy and timid puppy I got him at 8 weeks." The best advice anyone could give is to find an honest and knowledgeable trainer so that you can manage the dog to the best of his and your ability. The aggression is not protection -- only his self protection , fear . He is what he was at 8 weeks .
unfortunately you have a long and hard road ahead of you -- you must be realistic "he quickly reacts by barking and growling especially to children. There is no small children at home so he does not have much socialization with them"
prevention - prevent liability to yourself and danger to children --