Muscular dystrophy is not a single diagnosis. There are many varieties. It appears that your son who is now 14 years old has the disease since 8 years of age is suffering from Baker's muscular dystrophy. These disorders are primarily genetic and unfortunately no specific treatment exists as of now. Efforts are on for gene therapy. You continue with regular exercises. Bakers Dystrophy (if this is the case with your son) has a very slow course and he is likely to continue to do well.