I would get a brush that looks different than any other brush you've had if possible... and do not try to brush him with it
then I would start desensitizing your dog to the brush. For example, praise the dog when you get get the brush out and he ignores it. You can't start to close to him if he reacts. Get it out and set it by you on the couch or something and ignore / discourage any behavior that isn't calm. Call him over and praise him for not having a problem with it after a while. Gradually go to where you can have the brush right next to him, and he ignores it (praise like crazy for any improvement/wanted behavior). Just like Sweet posted... Eventually you can gently stroke the hair with the brush and treat, and slowly work towards being able to brush your dog You have to make sure your timing is right, so that you aren't rewarding your dog for growling or biting the brush.
Always end on a positive note, and make brushing as fun as possible. Brush
gently. Smearing peanut butter, or letting him chew on a bone, or something similar may help in distracting him from mouthing the brush, so that you can brush. Its a good idea to start working on this now, for a couple of minutes a couple times a day, and you'll be brushing by summer
Desensitizing is a long process, there is no easy fix
I hope that helps a little. There are comfort wraps and aromatherapy items that
may help if you find you need a little additional convincing that the brush is not the bad guy
Both my GSD and my brothers GSD LOVE being brushed... I hope he learns to like it too