The Terminator star has shared a photo with his 19-year-old son, saying how proud he is of Joseph and how much he loves him.
The pair were enjoying a day out at Oktoberfest in Munich wearing traditional lederhosen, and raising steins to their father-son bonding session.
"I'm proud of you and I love you!" wrote Arnold
"Happy birthday, Joseph. Great student, great athlete. I'm proud of you and I love you!" wrote Arnold, 69, on Instagram.
The actor has said little in the past about his relationship with his son, whose mother is Arnold's former housekeeper Mildred Baena.
Mildred worked at the Schwarzenegger household for 20 years but left her job when she found out she was expecting Arnold's baby. She gave birth to Joseph in October 1997. One month later Arnold's then-wife Maria Shriver also gave birth to the couple's fourth child, Christopher.
The actor with his girlfriend-of-four-years Heather Milligan
The Hollywood star maintains that he didn't know about Joseph until the little boy was seven or eight years old. He has since cared financially for Joseph and his mother.
Last year Arnold, who separated from his wife Maria in 2011, opened up about his "fantastic" son.
Arnold with his ex-wife Maria Shriver
"He's terrific and he totally understands the situation," the actor told SiriusXM host Howard Stern. "So, it all has worked out… It's a very tough situation for him. It's a very tough situation for my kids, very tough situation for my family. It was tough for everybody. But it has happened and now we have to figure it out, right?"
"I apologised to Maria. I apologised to the kids," he added.
Arnold has been dating Heather Milligan for the past four years.