Just adding my two cents.... Obedience is a valuable tool for any dog and I strongly advocate it all the time. However, obedience is not the cure all for a dog that has issues with other dogs. Exactly why does the GSD/mix have issues with "young" dogs? Has one attacked him at one point or annoyed him to the point he felt he needed to put the dog in it's place? Whatever the reason, it doesn't matter. What does matter is that your dog "does have issues with younger dogs" and you know it. Knowing your dog has issues means you need to be more responsible and on guard all the time when your dog is in public. This dog should have NEVER been off leash around other dogs. If it needs to run off leash you need to find a place where other dogs do not go. Take a walk in the woods instead of going to the park.

My suggestion is to work with a trained behaviorist if you want to get the GSD/mix to be able to assoiciate with young dogs at some point in his life. Trying to do this on your own is not an easy situation for the normal dog owner to deal with. In the meantime, exercise the dog in a secluded area without other dogs around or muzzle him and keep him on leash and under control at all times in a public area.