Do you have training classes in your area? You might consider some basic or introductory agility or rally obedience classes if they are available in your area. Controlled introductions and a "fun" activity such as agility or rally obedience might help your dog develop his confidence amongst other unknown dogs.
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