You find this in the mailbox
This will probably get him nervous, and will have to assure him there is no draft at the moment (emphasis on 'at the moment'). Theoretically, he could claim he is a Japanese citizen (which he still is), but that of course would not be a wise or pratical choice as he's pretty set here.
How'd they know? Well in this state at least, if you want to have the state issue a learners permit to you, you have to agree to the DMV releasing to the SSS, information on your B'date. Wonder if soon to turn 18 girls also get this letter?
Postscript: Opened the envelop, finding not a form to fill out and mail back in, but noticfication he's already been registered!!!