I'm unsure as to why my 9 month old pup has suddenly started to attack other dogs. Our trainer says he doesn't see any aggression in him. Any advice?

Hi. I had a question about my 9 month old black lab. He's been the sweetest puppy, other than his normal bad manners that need to be corrected. However, when I brought him around my brothers yellow lab for the first time he out of nowhere attacked this dog. Now he had been around 3 other dogs before this and was fine with them. He went for training and I spoke to the trainer about his aggression problem and he said he didn't see any aggression in him and proceeded to teach him basic commands. Well when I brought him home he immediately attacked a puppy. And is still attacking the other dogs as well. These dogs do absolutely nothing to him, just simply walk by him. I'm scared to death I am going to have to put him to sleep because he growled at my 3 year old cousin last week. I don't know what to do anymore. Any advice would be greatly appreciated. Thanks

Ashley