I've read all other responses, and I agree with trying to stimulate him without pushing him. I have thought two more suggestions:
#Find what motivates him
Find what motivates him
Explore the player, presents him various situations and try to figure which themes can appeal him. Romance, humor, adventure, violence, even sex. If you can find things that appeal him, you can use them as hook of roleplaying.
i.e.: In the tavern a sexy lady approaches and flirts with him. If he responds positively, give him the opportunity of trying to seduce this or other women.
If he does not show interest, try with a thug that brags and taunts him. Maybe he enjoy interchanging insults before fighting him. This could be a starter for roleplaying. Didn't work? Try finding something else.
I think Asperger syndrome people have narrower interests, but finding this interest can be the key to open him to the experience.
#Give him objectives and personality traits
Give him objectives and personality traits
Or better, try to find that with him.
I have a player which also has roleplaying problems. His main problem is to find what his character want to do. If he has clear objectives he can figure better what to do, and then interact with NPCs to find it.
It also helps giving the PC a clear behaviour pattern or personality traits. If he knows his character loves the knowledge, he will do anything, including roleplay, to find books or get the knowledge from other NPCs. If he knows his character is proud, he will try to seek the glory and defend his honor.
Having clear guidelines of what his PC wants and how he behaves (with extreme personality traits if neccesary) helps my player a lot.
#Find help with your players
Find help with your players
Pick a player with good roleplaying skills and ask him to chat with him in character, maybe while you are involved with other players scenes. Maybe he can talk with him with less pressure than with a NPC, or maybe he is more comfortable when the focus is the others players.
But make sure the player you pick has empathy and patience, and does not expect the same level of interaction and roleplaying from the others.