@fredsbend m.youtube.com/watch?v=pm6df_SExVw please see 1:53. In vacuum, air can escape quickly through several orifices, simply due to the vast pressure difference.

I really like the effort put into this answer, and it was eye-opening for me in several ways. However, I still gave a downvote because there was still a big issue unresolved: "closed system, if you hold your breath. As long as you don't try to breathe there is nothing forcing the air from your lungs...Absolutely none of these would happen in the vacuum of space". Actually, air would still get out. There are more orifices other than one's mouth, and they can never be all closed perfectly tightly, especially since the man is naked.