Last May, a teary-eyed Kimmel revealed during his ABC show that Billy had open heart surgery at three days old.
It was necessary as his son has a heart condition and Kimmel noted his "pulmonary valve being completely blocked and a hole in the wall between the left and right sides of his heart."
That wasn't all that the funnyman and his wife, the boy's mother Molly McNearney, had to endure.
In December 2017, Billy underwent a second surgery at the Children's Hospital Los Angeles.
As a result, Kimmel has become a huge fan of Obamacare. He has slammed Congress for neglecting health care for all Americans. Kimmel's celebrity pals, who attended his wedding to McNearney, and many of his fans love him for standing up for what he believes in.
And Kimmel wrote as a caption to his son's birthday post this weekend, "A yr ago today, God gave us this tough little boy. Today, we thank the nurses & doctors at @CedarsSinai & @ChildrensLA who saved his life & those who shared thoughts & prayers.
"We cannot stay quiet. Please wish Billy happy birthday by REGISTERING TO VOTE! vote.gov."
That last line could raise eyebrows as some think Kimmel has gone too far in talking about his political views.
As RadarOnline.com reported, the host recently got into an awkward Twitter war with conservative Fox News anchor Sean Hannity after Kimmel made fun of First Lady Melania Trump's foreign accent on TV.
After a fierce back-and-forth between the two men, Kimmel appeared to cave to Hannity, admitting he was wrong to mock Melania, but sarcastically tweeting, "Mrs. Trump almost certainly has enough to worry about without being used as a prop to increase TV ratings."
Kimmel also wrote, "I will take Sean Hannity at his word that he was genuinely offended by what I believed and still believe to be a harmless and silly aside referencing our First Lady's accent."
Trump family friend and fan Hannity accepted Kimmel's explanation and the men called a truce in their feud.
But Kimmel obviously hasn't stopped caring or making political noises on social media, as his son's birthday post shows!
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