Conscription for transsexuals in the US is especially fucked up. Transsexual women, regardless of their sex change status, still have to sign up when they are 18. (For you AMABs who are ”non-binary” or refuse to undergo sex reassignment surgery, that’s your own damn fault if you get penalized.) Transsexual men are not legally required to. Should he want to apply for college or certain governmental jobs but not want to conscript or out themselves, there is an alternative—the SIL, or “Status Information Letter”. You present the letterand no longer have to upload or mail out forms showing you had a sex change—it’s just one letter you submit with your proof of ID, and you’re covered.
Getting it takes time (usually about a month or so, longer with COVID). All you need is a letter from your doctor stating you’re receiving “proper treatment” (no, you don’t need to detail anything), and you should be good. Just mail a copy (not originals, they won’t mail anything back) of your birth certificate, photo ID, SS card, a letter from your doctor, proof of your name and sex changes, and the form stating why you seek a SIL.
They will only mail one copy. I recommend you scan a copy to keep digitally available, and keep the original locked away with your other valuable documents.