Jimmy Kimmel has paid a sweet tribute to his son Billy on his first birthday.

The 50-year-old presenter's little boy - whom he has with his wife Molly McNearney - underwent open heart surgery just three days after he was born last May, and Jimmy took to Twitter over the weekend to share how ''tough'' Billy has been, and to thank the medical team at Cedars Sinai and the Children's Hospital Los Angeles for ''saving his life''.

The talk show host also asked his 11.5 million followers to register to vote as a gift to Billy, to no doubt improve the healthcare system in the US.

Alongside a picture of his son in his highchair with a party hat on, surrounded by balloons, Jimmy wrote:''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! https://vote.gov (sic)''

Jimmy previously admitted he didn't want to ''get too close'' to his son after he was born with the congenital heart defect.

The TV star and his spouse weren't initially open with their feelings about the situation, but realised after the tot had undergone a second surgery they had shared the same fears.

He said: ''There were secrets we kept from each other that we revealed only after the second surgery.

''The biggest one was that, I think subconsciously, we didn't want to get too close to the baby, because we didn't know what was going to happen...I don't know if that's right or wrong or common or uncommon.

''But when I told her I was feeling that way, she said, 'Oh, I'm so happy you said that because I was feeling that way too, and I didn't want to express it.' ''

After having surgery, Jimmy - who also has kids Katie, 26, and Kevin, 24, with former wife Gina, and three-year-old Jane with Molly - revealed Billy was ''great'', though he still requires further treatment.

He explained: ''He'll have another surgery when he's around six or seven years old, but in the meantime, he's doing great.''