I had this problem with my last GSD. The way I handled it was as follows...
Show a treat to the dog and then take treat to top of stairs.
Come back down and go about your normal routine. Give the dog a chance to do it own his own. If he doesn't go up there, then bring dog to bottom of stairs and let him see you go up and get the treat. Take treat and put it away and let him see you do it. Wait around 30 mins and repeat above. I did that and it took about 4 times beofre he was going up and down them like clock work. If this doesn't work then you could always just sit at top of stairs and call for him and see if he will eventually come up. It could take an hour or so though. Goodluck....
Jag.. Jukk Last Van Hauser GSD, K-9
Sable..Dutch Shepherd, Explosives Detection Dog
Arko.. Bane VanDerHaus Bildtunderschrift GSD