I don't know how long you've been here, but I have a GSD mix that has some protective issues. My situation is very different however because I know that Kaiser's problem is very serious, and he has bitten. He is just over one year old, and only weighs 30lbs but he's nearly strong enough to pull me over. You mentioned that your dog is the strongest you've seen lb. for lb. and I completely understand 
At first I didn't think that the problem was major. He sort of growled at children and snapped once at my younger sister. We got a trainer that was trying to help us with basic manners like sit, stay, and loose leash walking. His behaviour got worse, the trainer's methods were proving counter productive. We tried a few other trainers, a behaviourist, and finally the behaviourist that my vet recommended to us. By this point, Kaiser had bitten and scarred my little sister's face and he was actively freaking right out merely at the sound of a child's voice. He was stealing food as often as possible, and would not allow anyone except my boyfriend and my dad to take things he wanted from him. Even then, it was never without a good fight. I spent over $2500 Canadian on trainers and behaviourists (within 4 months!) and no matter how great they seemed at the beginning not a single one turned out to be trustworthy, even the one recommended by our dear, trusted family vet.
I'm telling you this for two reasons. One is that I don't want you to waste your time and money on a trainer (or six) that can't help you. You only spent weekends and summers with him, are you in college? As a university student myself, I know that money can't be thrown around and wasted like that. You might get lucky and find a really great trainer that can do what you need, but be very careful in choosing. Your dog doesn't seem nearly as aggressive as mine is so he shouldn't need such intense intervention. Personally, because of the experience I've had, I would ask a vet's advice on what the root of the problem is, read up on it, and go to work. Save yourself the money from the trainer and when you're pulling out your hair in frustration because things aren't going well later, at least you'll have that to smile on
Believe me, it's much better than screaming in frustration AND realizing that you've spent more of your tuition money than you meant to.
The second reason I'm telling you all this however is that you need to seriously assess the degree of severity here. If it's not a big problem, there's a chance that you'll be able to bring him around with distraction tactics and such. It'll take lots of time, but it's very doable with a mild case. You know your dog best. If you are afraid he may bite someone, he probably would if given the opportunity. If that's the case, you might have to accept, as I have, that he'll never be safe in public and you'll have to put him in another part of the house if certain kinds of people come over, and you'll never be able to take him camping or anywhere public. My dog is perfectly behaved as long as he decides that likes you. But just when we're getting comfortable he'll snap or bite and remind us that we have to be constantly watching and on guard for the rest of his life.. I love him and he's worth it but it's a huge price to pay to live with an aggressive dog...
I hope for his sake that his situation is nowhere near as severe as Kaiser's, but you're the only one that can decide that..