As you've found out, there are many different disorders under the broad label of "muscular dystrophy." Some are sex-linked and some are autosomal; some are anticipatory - that is, they show up at earlier ages in succeeding generations; then, within individuals, there are differences - certain forms of MD can be mild. Once a specific diagnosis is definite, your family should get genetic counseling.
I'm sorry to hear they think your nephew has one of the more severe forms - one hopeful thing is that a lot of the research going on seems to be focused on the more severe forms. Contact your local Muscular Dystrophy Association group and ask for a subscription to "Quest" - it will keep you up to date on research and support. They even have a column for pen-pals, and those with the same forms of MD can link up to support each other and share how they are getting on with their lives. There are also ads for supportive devices, and information on funding and applicable legislation going on.
Best of luck to your family in a difficult situation.