I have raised a litter of Labs and had 6 of my own. Your dog has a lot of Lab but I doubt is full blooded. For one the legs look too short to be Lab. As far as Labs as a guard dog, I have yet to see an aggressive one. I had one that was a friend to everyone, another that hid behind me when any company showed up, and two of my current ones bark at strangers, go to the porch and keep barking until I show up or the stranger leaves. Discourages most but my regular UPS man has been accepted by them. If you want a guard dog that attacks it will not be a Lab. If you want an alarm bark Labs will do.
Don't worry about full blood in yours, he looks like a good dog that will be very loyal.
