It sounds like he's having trouble with his 'theory of mind' ie, how do I know what you're thinking. DS has Aspergers syndrome and this is his main area of concern. He just doesn't understand that we can't tell what he's thinking, and that he needs to use words to communicate things to us.
DS goes to an amazing speech therapist who works on this issue with him and also teaches us how to teach DS. Some of the things we do are,
Play 20 questions: Each of us takes turns thinking of an object and the other person gets 20 chances to ask 'yes no' only questions. This is great for making them realise that although the picture/thought is in their head, it's not in yours. It also helps them with describing objects when they need to.
Playing Guess Who? Games: you can get this game in a variety of characters. Our speechy uses one with people, animals, and a giant face one. The very concept of the game teaches that the other person can't see what you're seeing, and that you have to slowly answer their questions so they can eventually guess.
Play Eye Spy: Again, a game where people have to guess the thought that's in your head.
A game called 'Santa's Sack: in this game you hae to direct the other person to place presents on certain areas of Santa's present sack. You cant see each others board. In the end you both should have matching sacks.
Battle Ship game: Another great one for teaching that others can't see what you're looking at or thinking.
Once they begin to understand that what is in their head isn't easily read by other people they start to use the tools from these games to express themselves more. After six months of this DS made huge gains. He's a gun at 20 questions now! Although he still has trouble now understanding that we can't feel what he feels (his fingers were numb once ad he thought when I touched them I could feel the numbness) but its not a huge problem.
Perhaps visit a speechy, or if you already have one let them know this specific problem and see if they have any ideas to help.