'You are a champion': Arnold Schwarzenegger congratulates son, 22, on graduation from University of Michigan

He is the youngest of Arnold Schwarzenegger's four children with ex Maria Shriver and prefers to remain out of the spotlight.

But on Monday, Arnold shared a photo of Christopher, 22, to his Instagram to congratulate him on graduating with a Bachelor's degree from the University of Michigan.

Alongside a photo of Christopher in cap and gown mocked up to show him holding his degree and an 'I did it' trophy, the Hollywood star, 72, wrote: "Christopher, you are a champion and I love you", the Daily Mail reports.

Arnie continued: "I know your graduation from Michigan wasn’t the big celebration you dreamed about for years, but walking across a stage isn’t what makes me so proud of you: it’s your compassion, your hard work, your vision, your critical thinking, and your selflessness that make me burst with pride."

It's not known what Christopher's degree is in.

In September, his older sister Katherine, 30, traveled to the University of Michigan in Ann Arbor to spend Christopher's 22nd birthday with him and posted photos of the celebrations on her Instagram.