Patty Baena's Husband Had No Idea Her 13-Year-Old Son Wasn't His Until Last Week

For over 13 years, Rogelio Baena thought that kid was HIS . . . and he only learned the truth when the rest of us did.

But in an interview with "Entertainment Tonight", he relayed a message to the boy . . . quote, "I am your father.  That's all."

Asked how he felt when he learned the truth, Rogelio said, quote, "Angry!  Very, very angry!  Arnold Schwarzenegger, for me, was my hero.  Now, I feel betrayed."

We don't know how much contact Rogelio has had with the boy.  He and Patty split up just WEEKS after his birth in 1997 . . . although Patty didn't get around to filing for divorce until 2008.