OP, congrats. Your dog's story is the story of many, many adolescents -- cute as puppies but overwhelming to unprepared buyers once they got big and strong, so they end up in shelters as confused youngsters.
Take things slowly at home with your small dog, even though they got along well at the shelter. Read up about the "two-week shut down" as a method of introducing a new dog into a home -- it can avoid many problems (and some dogs need only a week).
I'm just curious why the shelter is not letting you have his AKC papers. He will be neutered so you won't be able to breed him.
There's supposedly a market for bad breeders to buy the papers and forge registrations for whatever litters they've got from some other unregistered dog under this dog--even though this dog didn't sire them because he's neutered, and they're not the former owner. I don't know the details of how it works, but apparently some shelter director busted people doing it.
When we get registered dogs in rescue, I take the step of calling the AKC and telling them to put a note in the dog's file that it's speutered and no future litters should be registered under it, but they make it really hard to do that (and they're so indifferent that I'm never sure they're not saying, "Yeah sure, whatever" to get off the phone without even updating the file).
The shelter mightlet you take a phone pic or photocopy of the pedigree (with a post-it to redact the former owner's name) so that you could research the dogs behind him, if you explain how helpful it is for health and training to know something about genetics in this breed.