"Now" in the sense that he should expect Americans to call him that, so he decided to use it as a nickname?

Or "now" in the sense that US government documents record him under that name?

What is "the immigration desk"? When he first arrived to the US? When he got his visa? When he got his citizenship?

When he enters the US (pre-citizenship, on his Chinese passport), how do they reconcile the name on his passport with the name on his US-issued documentation?

The Chinese nationals I know in the US did not have their name changed. Some do have English names, which they chose voluntarily as nicknames, but that does not affect their legal name.