My 21 mo old grandson doesn't like adults or children, and cries when they get near?

He is affectionate and engaging with his mother, father, me and grandfather. That's it. He will not warm up to anyone else and cries if anyone even looks at him. He will not participate in playgroups and doesn't like being around other children. He also has bad moods alot, wakes up grouchy and stays that way and has tantrums. His mother is a stay at home mom but she tries to take him to Gymborama and other type of play groups but he doesn't want to do anything. His father gives him alot of attention,too. He is very well loved and cared for. Also lately he has been hitting. What could be wrong and how can we help him? It is exhausting to everyone because he is hardly ever happy.