Jonnie Irwin said he was so proud that he could “burst” as his four-year-old son started school.
The snap showed Rex smiling as he posed in front of some trees in his black and yellow uniform.
“First day of school,” Irwin captioned the post.
“I’m so proud of him I could burst. X.”
Irwin’s fans were charmed by the photograph, with many saying that Rex was the presenter's “double”.
“So glad you can enjoy this special milestone,” said one fan.
“So over the moon for you, you get to see his first day, keep fighting stay positive,” said someone else.
“Aw what a handsome little man, it makes your heart want to burst,” posted another.
“I know every parent says this but for you Jonnie it is very, very special. Soak up every bit you can.”
One of the TV star’s followers wrote: “What a beautiful little boy, no wonder you are so proud.
“Healing thoughts and prayers to you Jonnie and your beautiful family.”
“A precious moment for you and your boy!” said somebody else.
Irwin, who is also dad to three-year-old twins Rafa and Cormac, was told that he had terminal lung cancer in 2020.
He went public with his diagnosis last year, telling fans he was determined to make the most of any time he had left with Jess and their boys.
The 49-year-old was initially given months to live and has said doctors did not expect him to see 2023.
Watch: Jonnie Irwin opens up about palliative care