My dog is so distracted in certain situations he completely ignores all commands and even treats, help!?

I have a 1.5 yo GSD who is a dream inside the house. He listens perfectly, never ignores a command, loves to learn new things, but as soon as we step out the front door he turns into a disobeying monster. I'm sure that he would still listen, if he was paying attention, but he gets tunnel vision and everything I say or do is outside that tunnel. Even high value treats (boiled chicken/ turkey pieces) don't get his attention. The problem is only getting worse and since he is a powerful breed I need to fix this, but I have no idea where to start or what to do.