Rice is Masculine, so why is this not "Der Mann isst den Reis", why is the article left out here
It could be, but this sentence doesn't use any article for the noun Reis. It could be:
Der Mann isst den Reis.
Der Mann isst Reis
Der Mann isst einen Reis.
Just like in English. "I eat (some) rice" or "I eat the rice" are both fine.