Hola. My second "son" Sherlock was a very frightened spoo when I got him. He separated himself in a corner and avoided me and my other standard Shasta. And he came from a well know breeder. His father was frozen sperm flown down from Canada to mate with a grand-daughter of Mikimoto on the Fifth. So, it was't a question of bad breeding. His breeder also owned about seven other standards so he had plenty of time with dogs besides his mother and seven siblings. As I know children who have come out of the womb jumpy and highly reactionary, I have written it off to being his natural temperament. And, that he was traumatized by being separated from his siblings and old "pack." As pointed out above, the 3-3-3 is a good guide. Like humans, with time and work, stressed out pups can become the best ever. I use to have a temper as a child. Now, I feel sick to the stomach if I even have to raise my voice to the dogs to get their attention. It just took a lot of love and support to get me to that point
My situation was different than your though as I did not have a child to worry about and could focus 100% on his fears (including butterflies and plastic bags.) I didn't work so I could keep him with me and introduce him to a lot of new situations one at a time. I made sure he got a lot of safe socialization with both people and other animals. He would actually go to a wine bar with me a lot where the owner let him hang with me at a corner booth where he could see the whole room but people couldn't approach him without bending down and crawling under the table a bit. There was always soft jazz playing, his corner was dark, and he had his "blankie" to lay on. Anyone who noticed and came over to pet him got the explanation of his fear of strangers and instead were allowed to take one of his treats, hold it under the table and talk to him, but then drop it and stand back up so he could get the treat and smell their feet (he has always had a foot fetish.) This worked. After a bit, he claimed the whole place as his and loved the the patio where anyone walking by would stop to reach over to pet him.
Now at five he demands everyone look at him. He is the biggest show off and a clown. I thank every day I have him although I still remember when I first got him how concerned I was that he would never fit in or if I had the ability to help him.
So, my suggestion is have someone do a temperament test on him, if you can, to first figure out if this is because of his transition to your home or a solo situation causing his behavior, or if he is just more on the skittish side (remember growling doesn't mean anger - it means fear.) Breeders of any breed of dog should be able to do this, or contact your local shelter pet adoption people to ask if someone there could do this for you. There are also people with pet therapy organizations who can assist in evaluating a dogs temperament. They mainly do this with matured adult dogs, but are skilled enough to give you a good opinion on a puppy. Here is the link to what organizations may be in your community.
AKC Recognized Therapy Dog Organizations – American Kennel Club
After having the puppy assessed, ask yourself the hard questions of if you have the time and desire to correct needed behaviors and get the proper socialization - if there seems to be any issues at all other than just initial transition problems. If the answer is no, that the steps to find the puppy a home better suited to his needs. And don't feel bad about it. It is just as important for the dog's happiness to find the correct setting as for you to find the correct dog. It may cause short term stress on you and the pup, but long term it is 100% the thing to do.
Because you have reached out here, it is obvious you have a caring heart and want what is best. There is a good possibility that what is best is your love and attention for this dog. It may also be a possibility that at no fault of yours or the puppy's, this was not the correct placement.
Regardless, keep us posted and don't feel afraid to ask anything. I don't know how many times I have asked poodle people a poop question and they still are happy to answer. Best wishes in your journey.