I believe that constipation can definitely be related to Addison's Disease. People with Addison's tend to dehydrate VERY easily, and when your're dehydrated, especially within your gastrointestinal tract, there will obviously be a tendency towards constipation. People with Addison's tend to consume more salt than those without and this causes the body to retain more fluids. Consequently, the intestinal tract may be "dry" or dehydrated, thus preventing the free flow of bowel movements.
I also have constipation, and had no problem with this prior to developing Addison's. Be sure that your nephew is drinking plenty of water, usually with a little sea salt in it .. do not avoid salt in hopes of preventing constipation. Salt is extremely important for people with Addison's. Just be sure that he's also getting plenty of fluids and good, healthy fats to keep the intestinal tract lubricated; walnuts, avocado, fish or flax seed oil, olive oil, nuts, seeds, etc. Good oils in the gut are KEY for moving bowels. Also is helpful to get plenty of fiber (whole grains, fruits & veggies,etc) And steer clear of foods that cause constipation : bad fats, junk food, refined carbs, etc. I'm sure this may be difficult at age 19, but it's worth suggesting to him.
Now is the time for him to take care of his body because trust me, he will regret it later.
And as much as you like his doctor, I suggest that you don't take his word as bible. Very few docs out there have adequate knowledge of this very serious condition. Most of what I've learned about this condition, how to manage it and how to live with it, has come from my own research, or from holistic practitioners or forums and blogs with others also living with Adrenal Insufficiency. And I've been at this for years.