Given the phrase "socially immature", another possibility that comes to my mind -- and of course it's only a possibility, I know nothing of this child or his family -- is undiagnosed Asperger's Syndrome, which could account for an excessively literal understanding of the "don't talk to strangers" rule. Asperger's kids can have real difficulty understanding that rules have exceptions, even if they're of normal intelligence and an age when kids can usually easily grasp that.