Eleanor,
You are right to be concerned about your nephew. His random behavior and problem with his legs is quite alarming but most likely NOT related to Addison's. Can you explain your concerns to him and get him to see a doctor?
Addison's Owner's Manual The Addison's Owner's Manual is an excellent resource for information about Addison's disease.
Please don't hesitate to ask if you have more questions! Can you get your nephew to post here?
:) Dusty