Brody Jenner Schools Photog, Caitlyn's NOT My Mom

Brody Jenner bristled a bit when our photog incorrectly referred to Caitlyn Jenner as his "mom."

Brody was at LAX Friday when Charlie asked his opinion about the latest unveiling ... Caitlyn in a bathing suit.

Caitlyn's son was appropriately complimentary ... and explained the drill when a parent changes sex. Things change and remain the same.

And one more thing ... if you had any doubt Brody and his brothers are estranged from the Kardashians ... listen to what Brody says at the tail end.