Encouraging your child’s sense of humour

How do we help a child who seems to have no sense of humor? Except for our oldest son, who will soon be entering high school, our entire family has always enjoyed the lighter side of things. Our younger kids are full of fun, and laughter regularly fills our home. By way of contrast, their big brother never seems to be amused by anything. He's always very serious-minded. I thought it was a phase that would pass, but to this day I can count on one hand the number of times we've heard him laugh. I'm concerned about how this will affect him socially. Even more importantly, I want him to enjoy life. Am I right to be concerned? Are there things we can do to encourage and help develop a sense of humor in him?