Im almost certain majority of my family members have some type of MI but I'm also certain that they would never seek help for it. I was laughed at and told to mind my own business years ago when I suspected my then 10 year old cousin had ADHD.

Sadly for many people to get diagnosed it takes a crisis that affects their life in some major way. I got my first diagnosis at age 6 because I was so far from a normal child it would have been insane not to take me to a doctor for my issues. Some people don't onset until later and for your cousin he's going to have to notice himself first.