Do you notice other signs of your dog’s issues? Unfortunately both of the experiences you wrote about are when others are dealing with your dog and in many cases, depending on the skill of the employees, different boarding facilities are able to put up with different things. The fact that the one place has a run for the dogs to play, doesn’t really guarantee that your dog gets let out to play there with other dogs. They might’ve tried it, saw the same thing Petsmart saw, and put him back in his own kennel run. Then just didn’t let you know that because they might’ve figured you knew that about your dog and most people when they board their dogs don’t really expect “play time” with other dogs…I know I wouldn’t.
I’m not sure why you have a muzzle for him…but I’d worry if I saw an 8 month old GSD and its owners were taking proactive steps by purchasing a muzzle. To me, it’s a sign that you’re unsure of how he might react in different situations. IMO, at 8 months of age a dog shouldn’t be mouthy anymore and should’ve been taught bite inhibition with regards to humans. The last thing you ever want is to put a muzzle on a dog around other dogs though. It makes them feel defenseless, and they’ll probably react more aggressively and differently than they would otherwise.
At 8 months old, he’s growing up and is going through some weird stages. Many dogs get a little bit weird around this time. So maybe that has something to do with it. At the same time…he’s maturing and his genetic personality will start to come out. So a dog that might’ve been fine a few months ago, might be showing some weaknesses in his genetics right now. Not saying he is, or he isn’t. But It’s not very shocking for me to hear that a GSD of that age isn’t acting like he did when he was 4 months old.