You mention only having your pup for eight days. How has the stool been for the entire 8 days? If there were no problems until the marrow bone, you might have something else going on. What was your dog eating before you got him eight days ago?
Sabra - female GSD - born April 30, 2012
Sargeant - rescued senior GSD male/foster failure - approximately ten years old (August 2013)
Max - rescued GSD - Dec 22, 2010 to March 7, 2012 - at the bridge
Sable - rescued GSD - at the bridge