Thanks for the info! Here are some additional techniques:
1) Share plenty of information about yourself and your own interests at random times when you are with your son as a way to help him stretch that muscle of listening to another person and taking in information. For example, perhaps you are in the car together and you see your favorite flowers, you could say "Ooh Lillies, they are my absolute favorite flowers!". You can even play with this by seeing if he retains that information by next time you drive that way saying "There's my favorite flowers again! I bet you can't remember what they are?'
2) Leave cliff-hangers during your conversations. For instance, perhape he is talking about cars and when there is a pause, say "You'll never guess what my fist car was?" That will help prompt him to ask personal questions of other people.
3) Use explanations, perhaps when you are talking about the food you like to eat, you can describe it in a way that's not revealing, and then remind him what he could say (e.g. "So I was eating my very favorite lunch the other day, and this is like the best ever sandwhich in the whole world"..........leaving a pause from him to be spontaneous, if he doesn't respond, explain "usually my friends ask me what the sandwhich is").
One game you could play is a word find game where you draw a large 10 by 10 grid on a posterboard and think of 10-20 questions that you can list down the side. The questions would be a mixture of personal questions that he can ask you and then facts about his motivations. Then write the answers inside the grid and fill up the rest of the spaces with letters so the answers are hidden. Make numbers one and two be about his motivations (e.g. "This was the name of the place the Titanic was sailing to when it sank", etc), then mix in some of your personal facts (e.g. "This is Mom's favorite color", "this is the best birthday present Mom ever got", etc.) So then you find the words together and when it comes to the personal questions and he is looking for the answer, you can remind him "Hey, I know a really easy way to get the answer......you can ask me!").
Another thing you can do is after you have explained that it's really cool and fun to have friends and part of being friends is being interested in eachothers stuff. You could brainstorm some opening lines he could say to people to start an interaction with him. You could even make a list and give him plenty of suggestions. It sounds like it's the starting an interaction that he's having a challenge with.
This is going to be so fun! Enjoy.