Some children in my experience take the first half of the year or so to feel comfortable in the new environment. Perhaps talk with his teachers and explain your concerns. He might need some ideas of things to say to get into play scenarios with other children. E.g "my names Tom, lets play with the trains together" or if he sees another child playing Lego "I like Lego too, can I play with you?" Asking for the teachers help to link him or introduce him to other children with the same interests can also help.