My scatterbrain teenager is a US citizen and has to re-enter the US in PHL early this afternoon, arriving from ZRH.
He forgot to bring his US passport, but took his other nationality one - never realizing he would need the US one for re-entry.
The other passport is from a country in the visa-waiver program, so we instructed him to apply for an ESTA online, which was approved promptly.
He was able to board his flight and is on his way.

But now what to do upon arrival ?

To fill out the I-94 is not so problematic, but with further questioning - and being an uncomfortable teenager - should he tell the truth and state he's a US citizen ?
Should he get in the visitor lane or the citizen lane ?
Does it make sense and is it allowed to show a copy of his US passport on his phone ?

I have no idea for him on how to tackle this and appreciate all help.

TIA !