I agree that you should deal with this now in a training approach, rather than after some incident as a last ditch effort.
The ideal boxer temperament as outlined in the standard:
These are of paramount importance in the Boxer. Instinctively a "hearing" guard dog, his bearing is alert, dignified and self-assured. In the show ring, his behavior should exhibit constrained animation. With family and friends, his temperament is fundamentally playful, yet patient and stoical with children. Deliberate and wary with strangers, he will exhibit curiosity but, most importantly, fearless courage if threatened. However, he responds promptly to friendly overtures honestly rendered. His intelligence, loyal affection and tractability to discipline make him a highly desirable companion.
You've got the deliberate and wary, but it sounds like the dog never had enough experience of the world to develop the radar to understand what constitutes being "threatened" and what are friendly overtures honestly rendered. That's probably more a socialization problem than a genetic problem, but chances are it's both to some extent. There are rescue dogs from puppy mills who have been poorly treated who have that ability to tell the difference between good and bad strangers, and otherwise well bred dogs with loving owners who do not.
Don't romanticize the "we feel safe because he'll protect us from a bad stranger" part to the exclusion of recognizing that he might also take a chunk out of the workman who steps outside for a cigarette and then comes back into the house 2 minutes later. Or the teenage brother of a child your child is playing with who comes to pick up the child and gets bitten for touching her. Or the paramedic who is trying to treat someone who is sick or injured.
Teaching the dog that there are people (male strangers) worthy of trust is an important gift and skill to give to your dog.