Like father, like son! Arnold Schwarzenegger is known for his days as a bodybuilder, and it’s safe to say that his kids inherited their dad’s love for fitness. The Terminator star took to Instagram on Monday, Oct. 2 to wish his son Joseph Baena a happy birthday — and the adorable father-son duo spent the day doing what they love most, working out in the gym! Watch the video below to see Arnold’s sweet birthday tribute to his son!

Joseph is Arnold’s youngest son, born just one month after Arnold’s second-youngest son Christopher Schwarzenegger. But unlike the rest of Arnold’s kids — 24-year-old son Patrick Schwarzenegger, 26-year-old Christina Schwarzenegger, and 27-year-old Katherine Schwarzenegger — Joseph’s mom isn’t his former wife Maria Shriver. Joseph’s mother is Mildred Patricia Baena, the Schwarzenegger’s former housekeeper that Arnold had previously admitted to having an affair with while he was still married to Maria — which is what led to their divorce.

Arnold and Maria tied the knot in 1986 but they announced their split in 2011, and even though it’s been six years since they separated, the estranged couple has yet to finalize their divorce. Arnold, who served two terms as the 38th Governor of California, opened up about the affair.

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Happy birthday sisssstaaah @katherineschwarzenegger

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“Without any doubt. Not only failure, but you feel like, ‘I’m to blame for it. It was me that screwed up,'” the 70-year-old said in a 2015 interview with Howard Stern. But even though he admitted that cheating on his wife was a big mistake, it didn’t stop him from being a good father to his son Joseph and the Schwarzeneggers have welcomed him into the family.

“[Joseph is] terrific and he totally understands the situation. So, it all has worked out… It’s a very tough situation for him,” Arnold said at the time. “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?”