My zippers have a mind of their own. However for a woman wanting to strike up a conversation with a man, it's a good pick-up line. But if a man should tell me, I'd probably wonder why he noticed. Personally I have never looked at a man's zipper. Nada, not once.
Simply walk up close enough to quietly tell him. He will appreciate knowing.
"Your tighty-whiteys are trying to get loose".
Hi guy, I know is hot in here but lets not carry this to an extreme