Spanish to English Art/Literary Textiles / Clothing / Fashion Underwear

Hi!
I'm translating a pitch with a script for an animated series, a comedy, and a male character in his forties/fifties can't find his 'calzoncillos'. I have no reference to the model (I don't know if they are boxer shorts, briefs, thongs or what), so I need a word that is more general. I was thinking of "underpants", but it would be great if a native speaker of American English or someone who feels confident enough could answer how natural that would sound. The pitch will be shown to an American audience. It's sarcastic humor (think 'Family Guy'). Then they refer to the "calzoncillos" and I was wondering about reference (singular or plural). The lines go like this:
-- ¿Viste mi calzoncillo? (In Argentina sometimes we refer to them in the singular)
-- ¿Tenés UN calzoncillo?
-- Es de mi viaje a X, es de (material). No puedo usar otro.

So, should the singular/plural issue be dealt with like this?

-- Have you seen my underpants? (Would Americans say 'Did you see...?')
-- You have only ONE pair of underpants? (I omitted 'Do' to make it more colloquial)
-- They are the ones I brought from X, they’re (material). I can’t wear any others.

THANKS SO MUCH FOR YOUR HELP!