However, when in new, exciting or unfamiliar situations, he may forget what he's learned and have difficulty regulating his behavior or modulating his voice.
There are growing number of great apps for i Phones, i Pad, and Android that young adults can use to assist with social interaction, including conversations.
When searching for Los Angeles, I found a number of groups for adults with ADHD, adults with high functioning autism/Asperger's, and others.
Meetup is also a great way to find local groups that share your nephew's interests.
Fortunately, there are a number of new social networking sites designed specifically for teens and pre-teens.
Imbee is one such site, designed for teachers, parents, and young children.
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.
The Whispy directory has a good list, with everything from dating sites for the deaf and those for individuals with neurological disabilities.
Depending on how your nephew's learning disabilities and social skills affect his relationships, he may want to consider joining a group locally for adults with LD.
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.