THANKS ALL FOR YOUR RESPONSES!
After getting past concern for the boy, my next reaction was sadness. At best, I suspected what most of you confirmed; Doug must be kept away from other people. At worst, this lovable, protective dog may be put down.
He was having so much fun while playing. The boys were batting tennis balls and he would retrieve them with his usual boundless energy. This is a dog that won't even 'play' bite, nor does he chase or herd running children. He does, however, seem fearful of strangers.
I did not witness the event and the boys weren't very helpful in the reconsrtuction. Indeed, they were oddly tight-lipped. We explained that we weren't trying to blame them, that we just want to learn from the incident so it could be prevented in the future. From their account, the boys were doing nothing unusual at the time. I am not surprised that Doug reacted to the helmeted person. You can get a reaction from him by walking oddly, putting on a costume, even donning a hat. I AM surprised that he bit the boy. I wonder if the boy might have made a threatening move -- in jest --, but I do not want to blame the victim.
I take to heart all of your advice. One complication is that we have two other dogs; a Brittany Spaniel and a Beagle. Doug is the alpha-male and he will not be understanding of the restrictions that are reserved for him. He'll have to get over it. In most every respect, he is the perfect dog. He is low-maintenance when you need to work on something else. His manners in the car are impeccable. He understands. or quickly learns, the verbal commands that make him a good companion.
Though he is young and has many years ahead of him, I believe he can adapt to a worthwhile life. He has become a risk, a liablity. He is a much bigger asset. I WILL be vigilant. Is there training that can minimize this risk?
Thanks again,
Mark