Almost all male citizens of the United States age 18 through 25 are required to register with Selective Service.
Men with disabilities MUST register if they are able to leave their homes, and move about on their own.
A man who has disabilities that would disqualify him from military service still must register.
Even though he is registered, he will not automatically be called to serve in the military.
If your family member doesn't register, there is a risk of him losing his benefits, like SSI or Medicaid.
When to register:
It's best to register within 1 month of his 18th birthday (1 month before or 1 month after).
After that, you can file a late registration up until his 26th birthday.
After turning 26, he is too old to register.
Here's what you can do:
Source: Selective Service System