Does it matter why? Bodily autonomy means bodily autonomy for everyone, regardless of whether you “understand” or or not. If he wants to be a more feminized masc, is it hurting you?

People modify (“deal a huge blow to” in your words) their bodies all the time to fit certain preferences. Be it tattoos, piercings (including earrings), cosmetic surgery or even stuff like body building.

It is not that strange that people want to express themselves with their body.

What do you mean by “huge blow to your body”?

Can’t give a general answer, but we can look to history. Consider the Castrati: removing the testicles before puberty prevented a boy’s voice from changing, giving them a high singing voice in adulthood.