The star left a sweet message and told Joseph that he loves him.
"Happy birthday Joseph!" Arnold said on social media. "Watching you graduate from college this year and seeing you follow your passions has been amazing. You are a fantastic son, and I can’t wait for our next training session. I love you."
Joseph, who is very active on Instagram, was quick to reply to his famous father.
"Thank you dad! Love you too and can’t wait for the next pump," Baena wrote.
Joseph and Arnold get together fairly often to pump iron.
The birthday boy is all about the pump and shares his gains with his thousands of followers.
The Apple Doesn't Fall Far From The Tree
Baena's most impressive physique pics are when he recreates Arnold's most famous poses.
Earlier this year, Joseph flexed his muscles on Instagram in a pose his pops perfected back in his Mr. Olympia days. He captioned the pic, "Had to take advantage of the lighting ? #classic #sidechest #trainheavy."
Like Father, Like Son
Joseph took the time to wish Arnold a happy birthday back in July and celebrated their relationship on social media.
"BIG happy birthday to the best training partner in the world! Love you dad," Baena wrote in the caption.
He's On Top Of The World
Joseph is on an upward trajectory at this point in his life. He recently graduated from Pepperdine University and he is an actor.
It's possible he could land the next big action hero role.
He is obviously very committed to fitness but has enjoyed the L.A. nightlife to celebrate and have a good time with his girlfriend.