5 Months isn't a long time to evaluate aggressive behavior. I would rather see the pup spend time with a professional trainer (who works with aggressive dogs), away from the family. That gives the dog have a 3rd chance.
Many GSDs test constantly for dominance and the worst of the lot (the most prey driven) can sometimes make excellent police dogs. Ck the yellow pages for a pro who works with problem dogs.
Hope all works out for you & your pup.