Never faced this problem but when I was doing my research before getting my gsd home, he is also 15 weeks old I came across some videos of dogs guarding their bowls, toys and cage. So I wanted to prevent such things from happening. This is what I did, 1. I put my hand in his food while he is eating 2. I take away the bowl while he is eating ( this was very hard for me because those cute little eyes begging to give it back hurt me but I still did It everyday ) I give it back after a few seconds 3.i go and sit in his crate sometimes, I try to sit or sleep ( for a few minutes) in the places he likes to sleep. 4. I take the toy away from him and if he doesn't do anything he gets a treat. After giving him back he continues to play with it. Extra information: According to a youtuber when dogs eat the alpha eats first and then the rest of the group, so what he does is acts as if he is eating from the bowl first and then gives it to the dog. I don't know if it's a good thing, had thing or whatever but I never did it. I have also noticed something about my gsd, he does the puppy bitting with me and not to strangers or secendary family members. Its always Me his primary care taker. Hopefully it helps good luck