As I posted in your other thread, if it's been going on for a couple weeks, you should take him to the vet to rule out any medical issue, he could have coccidia, giardia, parasites, or bad bacteria in the gut.
It could also be the food, GSD's can have sensitive stomaches. The BB may be to rich for him, the change to Kirkland may have upset his digestive track.
In the end, the vet is your best choice since this has been going on for to long
Danger Danger vom Kleinen Hain aka Masi
"Angel" Jakoda's Bewitchen Sami CD OA OAJ OAC NGC OJC RS-O GS-N JS-O TT HIC CGC
"Angel" Steinwald's Four x Four CGC HIC TT
"Angel" Harmonyhill's Hy Jynx NA NAJ NAC NJC RS-N JS-N HIC
"Angel" Jakoda's Jagged Edge