When he nipped the first time, he made an audible noise, half bark. I wish it was anticipation of playing, but it wasn’t. I have the number for the behaviorist and will be calling them to have them evaluate him to see if this is just his breeding or if it can be rectified. I do not want to put him down unnecessarily, hence why I came here for advice from other GSD owners. Comet, you have been extremely understanding and helpful, so thank you for that
What you're describing is typical of a dog that's resource guarding.
Two things make me think MAYBE this can be fixed: One, you've had the dog for 8 years now, and he has never, from what I gather, shown aggression before. And two, the injury was just a nip. If he were a seriously aggressive dog, the injuries would have been much more serious.
I had a female that didn't like kids. Fortunately, we didn't HAVE kids, and she was fine with us. Still, we worried. I talked to a trainer about it, and she said, "What your dog is doing is like a mother disciplining her pups. If she were a seriously aggressive dog, we'd be talking a trip to the hospital, and major stitches. (This was after our neighbour's daughter had been nipped on the face for trying to hug her. The nip left a red mark on her skin, and drew a drop or two of blood from her lip.)
The down side of this is I've dealt with Akitas. I worked for someone who bred them. One of them, the stud dog, was very mellow and laid back. There were two sisters that were seriously dog aggressive, and nearly killed a corgi when the dog accidentally got out of its kennel. Another pair of Akitas had to be separated when one of them began to resource guard its food.
None of them were aggressive towards humans, though. But some dogs (see above) are fine with adults and aggressive towards children.