Depends on how old he is.  10 US Code Section 246 defines the Militia in 2 parts:
1.  The Organized Militia, which consists of the Army, Navy, Air Force, and Coast Guard National Guard members, both male and female.
2.  The Unorganized Militia, which consists of all males between the ages of 17 and 45 and are either citizens or have stated an intention to become citizens.  Women can volunteer, and veterans aren't subject to the age limit. Females can volunteer, as can men over 45, but it isn't mandatory.  So, if you're under 46 and aren't in the National Guard, congratulations: you're in the Unorganized Militia.  It is your duty to be "well regulated," which means in possession of a weapon that is well-maintained and you're trained in how to use it.  We don't enforce that except in times of war.  It is this part of the Federal Code the allows involuntary conscription, aka "the draft."

It's your duty to own a gun.

https://www.law.cornell.edu/uscode/text/10/246