Many people with autism exhibit what is called Theory of Mind behaviors. This means that they lack the ability to naturally realize that other people have thoughts and feelings that are different from theirs. They lack the ability to take on the perspective of others or to empathize.
Most people learn this naturally through social interactions through the natural course of development. People with autism need to be taught about the feelings and perspectives other people have.
One of the larger complications this leads to is social understanding. The child with autism may not recognize the subtle cues of facial expressions, vocal tones, body posturing as they relate to the expression of feelings. Not to mention the use of language and its subtle nuances, metaphor and idioms. Children with an ASD more often are very literal cocrete thinkers.
How can I help this child? There are several ways to start. One might be teaching using cue selectivity to teach the subtle change in facial expressions. Using a hand mirror begin by making faces with your child, identifying the facial feature and pairing it with an expressive emotion. For example, when I frown it is telling you that I feel sad, make the frown face exaggerate if necessary, trace the curve of your lips, draw the curve of the frown on the paper onto a blank face. Require or help the child make the same facial expression, draw the facial expression, write the word. Practice it daily and begin adding additional exercises to it, stories about sadness, role playing, and try to help your child identify situations in which he has felt sad and write a personalized story about it, make it real.
Remember to focus on one cue at a time and then gradually build in one more cue at a time until your child has a whole picture of a person with the sad face. Focus one one cue at a time and pairing it with the emotional content is imperative. Keep language concise, clear, and precise. Utilize multiple modalities to engage the child into learning.