First off, your dog is gorgeous! But regardless of breed, your dog is telling you by his behavior that he's a dominant dog who is badly in need of training. Being protective of you, his master, is not uncommon in a SH, nor is it necessarily bad. But a strong-willed, powerful dog with protective interests NEEDS a strong alpha -- meaning you need to have complete control over a dog like this, or you're setting yourself up for trouble, and possibly a lawsuit.
If you can put in the time and work and treat him like a dog, not a person, you can work with all the issues you mentioned. I have a hundred pound mal mix and a week after taking her home, EVERYONE was telling me to "get rid of that beast because she'll bring you nothing but grief." She was strong willed, dominant, protective in the extreme, etc.
Today, she's completely off leash trained, and a certified Therapy Dog. But I was in a situation where I could afford to hire a terrific trainer with experience dealing with mals (he owned three), I happened to find a trainer who was more interested in my dog than my money, and he taught me how to become a strong leader so that I was running the dog, not the other way around. And I can tell you it was a lot of work. Totally rewarding, and I wouldn't trade her for the world, but it wasn't just about loving her, it was about working with her day in and day out, providing discipline and plenty of exercise, socializing her constantly, taking her for long walks, practicing commands, and working with her until she grew into the dog she is today...more Lassie than Cujo, I'm grateful to report! : )
If you decide to keep the dog, you NEED to invest in training. And you need to practice that training every day until your dog listens to you consistently, stops pulling, and follows your commands even with heavy distraction. It's okay if your dog is protective, but YOU should be able to call him off, or put him in a sit stay that he doesn't move from, no matter what else is happening. If you can't do this, you need to train like crazy until you can.
At five months old, you can still socialize this dog -- I got Jessie at 10 months, and she did not like or trust people at all, and today she visits nursing homes and plays with the residents. Your dog is young enough to fix a world of problems. But you need help, because this is a dog that needs an experienced dog handler, someone who knows how to deal with these issues and help the dog work through them.
So basically, you can either find this dog a home with an experienced dog handler, or become one. The first step is finding a quality trainer who is experienced with dog issues like these. We're not just talking about learning sit and stay and down, you want a dog trainer who knows how to handle dogs with issues. If money is an issue, perhaps you can work out some kind of payment plan. But you need to do this ASAP because aggressive behavior will escalate, and it gets harder to fix the longer it goes on.
In the meanwhile, be sure to get this dog a LOT of exercise. Running, walking, chasing a ball or frisbee, agility, whatever you can do to keep this dog active will help you with at least some of the problems. You know what they say...a tired dog is a good dog!
You also want to work on socializing the dog, but to do so safely, you probably want to work with a trainer so you can learn how to socialize your dog in a way that doesn't put anyone else in danger.
You CAN learn this stuff...most "experienced dog handlers" are former regular people who somehow ended up with a complicated dog and were forced to become "experts" in order to make the situation work...but unless you're willing to commit to becoming an experienced dog handler, learning training, socialization, setting limits, taking charge, how to "read" your dog, etc., your dog will have a better chance of having a happy life (not cut short by someone putting the dog to sleep because of a biting situation) if you find the dog an owner who has the experience to work with him.
You have my sympathy on this -- I know how hard it is to love your dog and not know if you're going to be able to make it work. I guess you have to ask yourself if you have the resources to do this -- financially, time wise, the right situation (a place where there aren't others who can get hurt, i.e. small children or others who elicit an aggressive response from the dog), knowledge wise (and/or access to experienced dog people who have handled problem issues), ability to become a strong alpha to this dog, etc. -- and then figure out what's best for the dog.
I wish you lots of luck! For what it's worth, some of the best dogs I've ever met were "problem dogs" that someone cared about enough to invest time, energy, work, money and love in.
Keep us updated on how it all works out!
Jan