This is probably the most important command IMO. I agree with everyone that a long line would be the best way to work with him, and work on it just a few minutes everyday, and practice whenever you get a chance. Also, try letting him play more after the recall more often, call them to you (long line attached just in case) give them the most awesome treat possible and release immediately. This way, coming to you means treats AND they get to keep playing. I have learned to try think of it like a dog might.....outside is fun fun fun, when they are called, fun is over and they would rather be doing other things. So, if they get the very tastiest of treats when they come for a recall, AND get to go back and play, it is a win win situation for the dog. Dogs are gamblers, if that treat is good enough in the beginning, they will remember they have a shot at chicken or liver or whatever they drool over. Also, when you go back in the house, give them another reward, I have some treats that sit by my back door. All fosters learn that coming in the house means they get a treat. coming to me means a treat and 70% of the time, it also means they get a treat AND get to keep playing or get to play a new game (chase is a great one when teaching re-call, but remember, the game is to chase you....not the other way around)
One other tip, when you call your dog, turn and walk (or run) the direction the dog needs to go to come to you. This becomes much more intersting for them and it looks like you may be going somewhere fun and be starting a new game.
At 9 months, your GSD is really entering a stubborn stage and I think it is even more important to demonstrat how much fun you are.
I do use an e-collar with Ava to proof now that she is off a long line, and it is an option for you, but I would lean more towards Lou Castle's theory and protocall than Leerburg. An e-collar can be very effective, but I feel it is worse than worthless if not fitted and used properly. If you want to use one, find a trainer that uses one in conjunction with positive training to help you get started, drop me a PM if you want some additional information on this and I can try to send you in the right direction.