Protective instinct/guarding behaviors commonly manifest in puppies that young in some of the more defensive molossers (filas, CO, CAO, CAS, Tibetan mastiffs, etc.). It's not surprising and pretty well correct for him at this age.
Gilles, you should definitely not allow him to get away with behaving like that. CAO are very independent thinking dogs. If he does not trust you to make a judgement call on whether someone is a real threat, you have a big problem. He will decide for himself, and you won't be able to control him if he decides someone needs eliminating.
If you ever get into a poop-hits-the-fan situation with him and can't pull him off, wrap the leash around a light pole, tree, whatever is handy let him pull against that until he tires. You might consider upgrading his equipment too. My comparatively shrimpy borzoi has snapped well made dog leashes in half lunging at coyotes. Look into the leads made for horses. They're made for 1200 lb animals and really comfortable on the hand too.
I'd also look into harnesses and tools that make it easier to control him if he does respond like that again.
Keep doing obedience work. Don't. Let. Up. Teach him a bombproof "Leave it!" or "Aus!" He needs to learn that when you stay stop, he STOPS. Let me know if you need help with this.
Avoid getting physical. Especially physical corrections with a dog like him. They are wicked smart. He already knows he's stronger than you. If you correct him physically he will learn that human-dog physical corrections are okay. He needs to be managed and you need to lead him in a way that he understands you're smarter than him and that you control all the good things in his life so that he will want to look up to you for guidance. It's VERY difficult to accomplish in a dog like him which is why they are not for beginners.
These dogs will love you with all their hearts and they would die to protect you, regardless of how much respect they have for you. That's why you need to make sure he is socialized enough to recognize a real threat, and that he respects you enough to take guidance from you if he misjudges a situation.
ETA: and a LGD can demonstrate true protection at a very young age. Renee experienced that when she and Kharma were ambushed by two armed men, and Kharma was only 7-9 months old (I can't remember which exactly).