They may recommend that your nephew work with an Occupational Therapist or school counselor on a social skills program.
You may consider checking out kid-friendly versions of adult websites, such as Teen Second Life, or you may want to stick with websites that are especially for kids.
Meetup groups exist for just about any topic or interest you can think of — from monthly wine and cheese parties to knitting clubs to hiking groups or Scrabble players.
Your nephew is sure to find at least one group in his area, and it can be a great way to meet people and make new friends.
There are also sites affiliated with children's products and entertainment, such as Disney's Club Penguin that have online games, chat, and social networking features.
Club Penguin is free to join, but advanced features require paid membership, so this may be a consideration.