I think the test your trainer performed was maybe done a little too early in his life with you. One month is in my opinion not long enough for the dog to realize that he is staying around permanently, let alone, take over the work of protection for the family. Boxers take a long time to mature. I have a show line Boxer who is very stable and a VERY good watchdog, and will respond to aggressive actions by people, with the same. As a pup he was always doofy, but he always reserved his judgement on people (this is what is talked about in the breed's characteristics). To illustrate, when a stranger enters my property he very vociferously barks and is not interested in being their friend- unless I tell him that it is okay. Once my neighbor who is a not so good person (that is just about the NICEST way I can put it), got ticked because my dog just stood next to my side even when he called him- so what did this numbskull do? He made a jerky movement to try and scare the dog. Result? My dog lunged for him. I quickly got him back into heel, and the dog has never trusted him since. When I came home once, my father had been home before me and left before I got home with my Boxer. As I entered the house, I unleashed him, and he immediately proceeded to check out each and every room. This is not something I taught him. He is one stable dog. He is also very confident, and with me quite the puppy. Boxers are not GSD's or other well known protection dogs. They are dogs bred to be close to their family and to handle any threats that they deem a danger to their family. The book "My life with Boxers" written by the greatest influencer of the breed Friederun von Miriam-Stockmann, has a very lucid and compelling description of the Boxer used diring WW1. Now, yes, the breed has lost, in the show world some of it's nerve, but not all have.
I have added here, an excerpt from her book:
"... Immediately the Boxer leaped into the brush. In just seconds the woods were full of shouts. Quickly, a second patrol came over. A search of the woods turned up the culprits. The first one had a bad bite on his upper arm; the second had a facial injury; the third had a badly chewed rear end. Later investigation revealed that the first had tried to use a club on the dog, the second had tried to shoot it. When the third saw his friends being overcome by the Boxer, he decided to climb a tree. But the dog pulled him down. All three were laying on the ground when the patrols arrived, and the dog stood calmly among them. Normally, if the dog or soldier were not directly attacked, the dogs did not bite. Today, dogs are trained to attack the arm, but at that time the training procedure was to atttack with a leap to the back of the neck."
Here is a description of the character of the Boxer:
"In the Boxer, the character is of the greatest importance. Loyalty and faithfulness to his owner and household are inbred traits, and he is renowned for his unfailing vigilance and dauntless courage. These make him unsurpassed as a guard and protector. Within the family he is docile and reliable, a great friend to children, quiet and trustworthy, even with strangers. He is joyful and restrained in play with his friends, but can be terrible when roused to anger......The Boxer is the soul of fidelity, bravery and honesty. The worst faults of character a Boxer can show are viciousness, treachery, unreliability, lack of temperament and cowardice."
Then, another passage entitled: Thoughts on watchfulness and the drive to protect:
".. Guarding is nothing other than defending their territorium- in this case the home- against all intruders, even the friendly ones. No dog is interested in defending a strange place that has no ties to him. Of course a biting dog - especially a fear biter- often attacks everything that comes near. But that type of guarding is usually worthless because of the constant barking that goes with it.
Tests using a fake burglar should not be used. He might indeed be attacked by some dogs that are over-aggressive but perhaps not good watchdogs. Either the dogs recognizes the burglar or the situation from earlier experiences, or the person arouses the dog because of some unusual appearance, or action. In which case the dog attacks mainly to protect himself rather than a foreign house. Testing for the dog's protective qualities should only be done when the dog is at home, without the owner being present of course. I believe very much that character is an inherited featur, just as much as appearance. But many time we overlook that fact that character is often polygenic as appearance. Many inherited factors make up both. And the complete dog must be a balance of these. The head alone does not make the dog, nor does aggression-impulse alone make the good Schutzhund. A good protector of the master does not necessarily make a good watchdog..."
Frau Stockman was the pre-emminent developer of the Boxer as we know it. She singlehandedly protected the breed from dissolution in bad times in Germany. This book is a must read for ANY Boxer owner.