I know a lady who had a GSD from a pup and her and Cola used to get along great, then when she was about 10 months old she started attacking all the dogs in the park (Cola included) and nothing worked so she has started taking her to training classes or a behaviourist I dont rightly remember ... We dont really see much of her now as she tends to walk the dog very early or late as many people have complained but she seems to finally be getting better but its been a long and slow process.

I know every dog is different but you really need to work on him now as it may well get out of hand and they need good social skills ... please look for behavioural advice from someone qualified.