Arnold Schwarzenegger's love child grabbed a massive sword and a leather loin cloth ... and sent one helluva Halloween message to his dad -- by dressing up as Conan the Barbarian.
14-year-old Joseph Baena hit up Frightfest at Magic Mountain in California last night ... donning the costume made famous by the father who kept him in the dark for more than a decade.
Cool tribute? Cry for attention? Both?