Arnold Schwarzenegger and his son, Joseph Baena, celebrate at Oktoberfest

Arnold Schwarzenegger has taken to Instagram to tell his 19-year-old love child that he loves him.

The Terminator star posted a sweet photo of himself and Joseph Baena as they celebrated the teenager's birthday at Oktoberfest in Germany.

"Happy birthday, Joseph," Schwarzenegger wrote in the caption, "Great student, great athlete. I'm proud of you and I love you!"

Schwarzenegger made headlines around the world in 2011 when he confessed to fathering Joseph Baena with his former maid, Mildred Baena.

The confession ended his 25-year-marriage to Maria Shriver.

Speaking to Howard Stern last year, the legendary action star admitted the affair was "without any doubt the biggest setback and the biggest failure" of his life.

"And not only failure, but you really feel like, 'I am to blame for it. It was me that screwed up'. And you can't point the finger at anyone else. So yeah, I was disappointed, but it's always easy to be smart in hindsight. The thing was, 'How do you now make the best of that?'"

Schwarzenegger and Shriver tried to salvage their marriage by going to counselling, a move the actor clearly regrets.

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"It was the biggest mistake I've ever made because (the therapist) was so full of s**t," he said.

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"He said more crap and more nonsense ... It was absolutely, totally counter-productive to our future relationship or to any hope to get together or anything like that. It was just nonsense talk. It was the biggest mistake.

"Maria talked me into it. I went, and I felt instinctively maybe I shouldn't go because I know I screwed up. I don't have to go to anyone to have him explain to me anything."

Schwarzenegger said his love child, Joseph Baena, has handled the drama well though.

"He's terrific and he totally understands the situation. 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?"