gs aggression to dogs

Question
Adopted 4 mo,maleGS from a litter of 8.He was aggressive with other dogs in neighborhood (while on walks) but not our minpin at home.He is 16mos ,neutered, went to puppy school and needed separation due to severe aggression. He is not food aggressive, great with family and 5 yr old son but very shy around strangers(doesn't growl or bite) When he sees another dog his fur is up,barking, exposed teeth,growling,w/ a full on attack to destroy

Answer
The problem is likely due to a lack o socialization before 12 weeks. Many people don't understand how important the period between 6 and 12 weeks is, including many breeders.  

I would have hoped the class instructor would have been more help. The key to most behavior problems is approaching things using the dog's natural instincts. Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog. Life is much easier if the 2 legged pack members outrank the 4 legged ones. You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. A good book could be a big help.  Look at the Monks of new Skete or Patricia McConnell.  Avoid Jean Donaldson or anybody else whose books is full of psycho babble.  

Do what you can of this:

''Elevation for small puppies: Sit on the floor and gently put your hands around your pup's middle, below his front legs, and lift him up. He is facing you. Hold him for 15 seconds. Repeat until he no longer struggles. If he is past 10-12 weeks, lift his front feet off the ground, but don't pick him up.

Cradling for small puppies: Hold your puppy gently on his back, as you would cradle a small baby. If he struggles, hold him firmly until he quiets for 10-15 seconds. With larger pups, you can do this as your sit on the floor, with your pup between your legs.

Quiet lying down: Place your pup on the floor on his side, with all 4 legs pointing away from you. Use your hands on his neck/shoulder area and middle, to hold him in this position. When he is quiet, praise him. Lengthen the time that you keep him quietly in this position. When he accepts this position well, handle his paws and muzzle, while keeping him quiet.''

The quotes mean this isn't my original work. It is copied from my Puppy Raising Manual. I have long used these or minor variations of them, and they are very effective. You may want to give him a belly rub while he is on his back too. Helps bonding. There is a big difference between him rolling over and demanding a belly rub, and you choosing a time to roll him over and rub his belly. The latter cements your place as pack leader.