If he is going to be penned or crated all day in the event that somebody comes to the door, that IMO is just as bad as being sheltered.
Besides being professionally trained, I work with him all of the time. Incidentally, I have walked the dog past the teen that he attacked many times before the attack. In his mind I guess that when he is walking with me, he has a job to do and that is to stay tight to me, look forward and walk.
Just put a leash on him. When someone rings a bell I either put a leash on him or lock him in a room
I doubt he was protecting you. There was no threat, you didn't tell him to protect. He ran out on his own to chase a teen of out of insecurity.
But let someone with more experience reply