Ask him what he would like to do and see if he comes up with any ideas. If not, give him serveral activities to choose from and tell him that he has to pick one. If he participates several times and doesn't enjoy it, tell him that he has to pick another activity to try out. Don't just let him sit around. Also, ask your doctor for recommendations for activities for young children with Aspergers.
Have you looked into any support/youth groups that is spacifically for kids with Autism?? I work in a center called "Hope For Autism" this is situated in Airdrie, North Lanarkshire, Scotland. Heres the link to their website http://www.hopeforautism.org.uk/. Good luck with your son, although be very careful not to force him too hard to socialise…it will take time. X