First of all, dystrophin is in the muscles. Many well-meaning people may want to discuss treatments with you that involve the brain, the backbone, or the nerves, but that is not where the problem lies.
Skeletal muscles are made of long fibers, surrounded by a membrane. Dystrophin apparently helps protect that membrane from tearing when the muscle contracts. Without it, the muscle gets hurt. The body reacts to that the same way it would to a muscle that got hurt by outside forces: it releases CPK, and it replaces the muscle fibers by growing new ones. At first, the muscle fibers that break down in kids with DMD is continually replaced. But there is a limit to the number of times a muscle fiber can regenerate. After a while, the muscle begins to break down faster that it can be repaired.
What we can see under a microscope is that healthy muscle tissue over time gets replaced by fat. That is why at a young age, your child may look like a future line-backer, because the fatty tissue is making his calves almost pop out. That is also where the name "dystrophy" comes from: it means "incorrect feeding". You may also come across the term "pseudohypertrophy", which means "make-believe overfeeding."
DMD affects muscles, but not all of them, and not all at the same time. Doctors used to think that the course of DMD was consistent and highly predictable. They are getting away from that notion now. It turns out that boys of the same age can vary widely in their abilities. And it turns out that the things you do with your child can hold back a next phase for a while.
By the time you notice a loss of function, it is estimated that about a third of the muscle mass has gone. DMD affects voluntary muscles first, the 434 muscles that you have conscious control over. And of those, the ones around the hips and the shoulders are the first to grow weaker. Over time, the leg and arm muscles also are involved, and the muscles holding up the backbone.
Later on, involuntary muscles are affected, too, like the muscles that make you cough and breathe. Heart disease is said to be a frequent complication of DMD. From what I understand, constipation caused by sluggish bowel muscles is about as common for a boy with DMD as for anybody who sits down all day.
One thing children with DMD apparently have more than others is seborrheic dermatitis, a dandruff-like condition of the scalp. They also show "flushing" of the skin a lot.
