A 5 years old boy shows no interest in other children and ignores adults other than his parents, even he is hard to make eye contact with people around him. He spends hours lining up his toy cars or spinning their wheels but does not use them for “make-believe” play. He likes watch the same movie over and over again. He rarely uses speech to communicate, and his parents state that he has never done so. He is “in his own world” most of time. Physical examination indicates that his head is of normal circumference and his gait is normal. What is the most likely diagnosis for the boy