Grab the loudest squeaky toy you have, stand inside the door, call out his name and squeak it. Make it fun for him to come inside. Reward him with the toy and say good boy " or how ever you want to praise him." After a few times, when he hears that toy; he will come running for the fun time.
"You could have the worst day in your life, but when you go home to a German Shepherd it always gets better"