Joseph Baena posts childhood photo with Arnold Schwarzenegger on his dad's 73rd birthday

Joseph Baena paid tribute to his dad Arnold Schwarzenegger on the occasion of his 73rd birthday yesterday, with a rare throwback picture.

Baena posted a shot of himself as a young boy cuddling Arnie, as well as another shot of the Austrian Oak in his pomp, with 'Arnold Is Numero Uno' on his t-shirt and a plate of fried chicken on his lap.

Read more: Joseph Baena looks just like his dad in gym photo

The shot is from the classic documentary movie Pumping Iron, which first introduced Schwarzenegger to the world as a young body builder.

“Wishing the Happiest Birthday to my Dad! You are and will always be 'Número Uno',” Baena captioned.

Joseph is the son Schwarzenegger had with his former housekeeper Mildred Baena, an affair which ended the Terminator star's marriage to Maria Shriver.

However, he now appears on better terms with his ex-wife, judging by a picture posted of his birthday celebrations.

He's pictured with Shriver, three cakes and their four children Christina, 29, Christopher, 22, and Katherine, 30, who is married to Chris Pratt, and 26-year-old Patrick, who posted the snap to his own Instagram page.

Schwarzenegger and Shriver split in 2011, at the close of his tenure as the Governor of California, an office he held for eight years.