DS is 3and a half. We have just been to nursery parents evening, and they are suggesting that he maybe needs to see a specialst. 
He is not good with children his own age, and is shy with strangers. At home he is very chatty with adults he knows well, and interacts with his little sister, although he ignored her for the first year.
He memorises books, or bits of tv or films and sits and recites them to himself, or to me. Sometimes he inserts himself into the story, or acts out one of the characters. He gets kind of stuck in a loop sometimes and recites them over and over. He also loves trains, and talks about them a lot.
Is this in the range of normal behaviour, or should I be worried?