Awww, poor Scout! Time and repetition, it sounds like you are doing the right things to help him (down when strangers approach with whom he's uncomfortable is a god example). The more you can take him to safe paved places the more comfortable he'll be. Is his vet's office in a paved lot? Maybe he's associating pavement with being poked and prodded ...

He is, at 1 1/2 years, still a puppy in many ways. Don't give up or feel guilty, just keep on training. He may never be a happy-to-meet-everyone sort of dog, but at least he's good with children! That's more than I can say for some humans I know.

As with any "used" dog, you don't know what horrors he may have experinced in those first few months of his life. That may have a lot to do with his fear of people approaching, for example. But at least now he's got you to love him and try to help him, and an ever-exanding circle of "okay-by-him" people.