You mean his tax return, correct?  You should use the current mailing address that the IRS would use to contact him with correspondence or to mail him a check (if applicable.)  Otherwise, if there is a problem with his return, they would write to the wrong address, and he likely would not get their letter.

Thank you Tom for your response.  I do mean his W-2 form.  He only has a temporary residence in NYC for his internship and then he will move back to Illinois probably unless he gets a job.  If he puts the NYC address, come 2021 he will no longer live there.