The 69-year-old action star shared a beaming father-son photo from the world-famous beer festival, where they were celebrating Baena's 19th birthday. Both generations dressed up in traditional Bavarian garb and raised some huge mugs of frothy ale for the sweet snapshot.
"Happy birthday, Joseph!" the former California governor captioned the pic on Sunday. "Great student, great athlete. I'm proud of you and I love you!"
Schwarzenegger has been in Munich for a number of days, as part of his annual pilgrimage to the wildly popular volksfest. Last week, the Terminator star even got the chance to conduct the Oktoberfest orchestra at the Theresienwiese grounds, where the traditional festival has been held since 1810.
Baena isn't the only person near and dear to Schwarzenegger's heart who accompanied him to the grand festival. The actor's girlfriend, Heather Milligan, also flew out to share some brews.
Schwarzenegger has been dating Milligan, his physical therapist, since separating from his ex, Maria Shriver. Schwarzenegger and Shriver ended their relationship in 2011 after news broke that he had fathered Joseph 14 years earlier with the family's housekeeper, Mildred Baena.
Schwarzenegger has four other children from his marriage with Shriver, 26-year-old Katharine, 25-year-old Christina, 23-year-old Patrick, and 19-year-old Christopher.
Christopher just celebrated his birthday last week, and Schwarzenegger commemorated the occasion with a sweet throwback pic. The star captioned the cute snapshot, "Happy birthday Christopher! I love you and I'm so proud of you. I know this year will be your best yet."