How much do you exercise him? And mental stimulation. You have two high energy breeds mixed in there. And GSDs are intelligent too. It may be that he's hit his 'terrible teens', they seem to forget everything they have learned. Without knowing more, I'd suggest more physical exercise and some mental games too.
He's still growing up , dogs mature about 2 to 4 depending on breed so keep up with the training and perhaps if you can , do things with your dog that he has been bred to do , that he finds satisfying.