teaching Andrew to be more sociable

It is difficult seeing my little boy not playing with others when we go to the park. His brother is a social butterfly, playing with everyone, telling his life to everyone and making friends instantaneously. In the meanwhile, Andrew will play with sand by himself, he will go up and down the slide by himself and will not be in the least interested by the other children who are laughing and playing close to him. It is as if his brother had inherited 120% of my social skills and Andrew had inherited 5%. Sometimes, another child gets close to Andrew and starts playing with him. If Andrew is not too tired, he may let the child interact with him for a while. If Andrew is tired, he will grab his toy, run away and tell him to leave him alone.
Is being sociable overrated? I think we all have different needs of sociability, but at a minimum, we need some so that we can interact with others, have a few friends and be happy. Being at preschool with typical peers is SO important for Andrew, I cannot stress that enough. Having his therapist come along is SO important for Andrew! He needs to be encouraged to socialize, if not, he simply will not. This skills needs to become natural.