You have probably already done this, but have you approached the problem directly and asked him why he doesn't want to laugh?
Solid might be right or he may have been teased and bullied by his peers and sees his laughter as the cause.
Does your son have self-esteem issues? If so help him see his strengths, encourage his interests, and praise his accomplishments.
Our son laughs but it took him awhile to develop a sense of humor. If your son doesn't like to laugh you may have to be satisfied with a smile or small chuckle.
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Aspies, the next step in evolution?