It's not uncommon for the herding group breeds to be scared of strangers if their breeder is not paying attention to temperament. That "aloof or reserved with strangers" becomes "afraid of strangers". Since he is from a rescue, nobody was paying attention, unfortunately. The fear is likely genetic so can't be "fixed" per se, but you can help your dog learn to cope.
First, I would check out the Reactive Dog sticky thread. This might not be exactly your dog, but much of the training is going to be similar when it comes to counter conditioning.
Your dog is likely never going to like strangers, but you can work toward having him ignore them and be comfortable with the everyday stranger encounters, like passing them on a sidewalk. It will be slow progress, so patience is important, and you must work at the pace of the dog.
You might also consider looking up the "Muzzle Up" project, which conditions dogs to happily wear a muzzle. There are some times that our dogs have to tolerate strangers, like at the vet, and if your dog is one that might snap when afraid, the muzzle will protect the vet and the dog from getting a bite record.
First, I would check out the Reactive Dog sticky thread. This might not be exactly your dog, but much of the training is going to be similar when it comes to counter conditioning.
Your dog is likely never going to like strangers, but you can work toward having him ignore them and be comfortable with the everyday stranger encounters, like passing them on a sidewalk. It will be slow progress, so patience is important, and you must work at the pace of the dog.
You might also consider looking up the "Muzzle Up" project, which conditions dogs to happily wear a muzzle. There are some times that our dogs have to tolerate strangers, like at the vet, and if your dog is one that might snap when afraid, the muzzle will protect the vet and the dog from getting a bite record.