I searched "Underwear", "Underwear in the 1930s", "Mens Underwear", "1930s Mens Underwear", and a lot of other terms. The most I got was wikipedia (which explained a lot but didn't give me an answer), patents for different underwears, and advertisements from now and then. I'd greatly appreciate it if someone helped!
(NOTE) Pierre always dressed casually and even mote now after spending years in New Orleans. So please don't think that he wears fancy suits and stuff like that.
EDIT: Thanks guys! I got my answers! I can't believe how many people replied.
And I do know that there's no throne in France! I mentioned it at some point, and I think we're forgetting this is a Disney fanfiction. They made dogs talk, they made up countries. So I do know my history.