Best time to train...feeding time. Put however much kibble you feed him in your pocket. Give him some commands. If he doesnt listen, no need to yell at him. He just doesnt eat. Just walk away and ignore him. He will most likely follow you. Try again and repeat until he listens and then reward the crap out of him. He will eventually realise good behavior fills his tummy! Its an easy way to make sure youre not yelling and making bad memories. This period is all about over rewarding his good behavior and pounding that good behavior gets him good things into his brain. He is a GSD so this will be very easy.