It sounds as though your son has developed what the psychologist, Erik Erikson, described years ago as the most important achievement of the first year of life—basic trust. Trust that the parents will care for him, meet his needs, and love him. If you had not done a good job of meeting his needs, he would not appear so secure and so accepting of others. He seems to have done what, in a perfect world, we would like for all children to do—generalize that basic trust to other people. There are some developing societies in which this kind of behavior is very common, as though the children grow up expecting all adults to act toward them as their parents do.
But we don't live in a perfect world. Strangers not infrequently kidnap children in malls, and you cannot afford to allow him to be given over to them and held. If you see it coming, quickly suggest that it is time for you to leave or go to another store. And use distraction as a helper: hand him a favorite toy that you've been keeping back for just such a moment, and then move away from either embarrassment or danger.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.