How can we tell our child about his muscular distrophy in a truthful but not frighteneing way?

Gently and gradually. Children have the right to know when they have a problem, especially one as serious as muscular dystrophy. However, there are various types of MS that have quite different courses. Starting with validating the symptoms as "real" not imaginary can be a great first step. Making it clear they can ask questions and get real answers is the major goal.
Gently. Tell him he is special. His muscles are different and react different than others. And just like other people we are all different in our own way.