Walks? German Shepherds laugh in the face of walks! Take him on runs, or to a lake and let him swim. He needs to do physical and mental activities.

Are you tired after a walk? If you are tired, he will be tired.

We live in a 1 bedroom apt with no yard and a 1 year old female GSD that we have only had for 3 months (God only knows who owned her previously). She needs serious exercise at least 4 times a week, otherwise she is obnoxious and driving me crazy!
She goes on 3 mile runs with my fiance 1x a week, out to the barn and runs up and back the half mile drive way in front of the car 3x a week, goes on horseback rides 2x a week, and plays with my chi-weener-russel at home. Is she tired after those outing? Yes, for about 45 minutes and then you would never know she's been out.
I also work on their obedience most nights and they get mental stimulation trying to wrangle and kill the barn rats.

You can bring him up to Gainesville for a week and see how TIRED and QUIET he is when he gets done with our exercise routine!