GSD - Aggression Issues

[Jaxamon]

I have a neutered male, 1 year 5 months. He began showing aversion to strangers around the one year mark. During his first year we made a huge effort to introduce him to as many new people and environments as possible, as suggested by many of the numerous training books we read. I know how important socialization is for GSD and we must have messed up somewhere. His reaction to strangers is the biggest problem we are facing.
He is loving and affectionate to myself and my husband, his parents and my parents. There are a small group of friends who have met him recently and he has been great with, after a proper introduction. However, those are the exceptions. Strangers passing on our walks, entering our house, or even present in another house that we bring him to all cause aggressive behaviour in him.
His focus and attention locks on to them. He switches into a completely different dog. He attempts to lunge and attack them. He has bitten 2 adult males, and has pushed a child off of a bicycle.
Another issue is extended family members. They are willing to meet Jax and give him a chance, and I am looking for advice on the best way to go about introducing Jax to them. His previous behaviour around 2 family members hasn't been great; he lunged and tried to bite one (a male, who was just sitting in a chair as Jax approached him), and barks and nips at the other (a female, who is visibly frightened and nervous of him).
He is a big guy, over 100 lbs, and has a loud deep bark, and so it's understandable that people are going to be wary of him. He is jumpy and quick to react, extreeeemely alert to any sound and movement.
We take him to dog kennels and daycares and he is just fabulous (always conducting a behavioural interview with the kennel beforehand). We always are told by the staff what a great dog he is.
I am just really, very confused. It's like he has split personality. We keep an extremely close watch on him, and he wears a cage muzzle any time he leaves the house. I am still terrified that something is going to happen and he's going to be taken from us, unless we solve this issue.
We were working with a trainer in private lessons and completed about 2 months worth of training. Jax responded well to the commands and cheap tricks, but we felt the trainer was not what we were looking for in terms of the behaviour modification we are hoping to help Jax with.
Our area (Petawawa, Ontario) is extremely limited in regards to available trainers. We travelled to Ottawa for the last one, and would be willing to do so again given it is the right trainer. If anyone knows of one and could provide contact info, that would be great. Any insight into working with a GSD would not go wasted either.