Justin Theroux shares warm birthday message for friend Jimmy Kimmel's son Billy on his 1st birthday

Justin Theroux on Sunday took to Instagram to share a warm birthday message for his friend Jimmy Kimmel's son Billy on his first birthday.

The 46-year-old star of The Leftovers shared a series of shots of himself holding the tot, who turned one on Saturday, wrapping up a first year in which he underwent a pair of surgeries to deal with a congenital heart condition.

'Happy ONE year birthday Billy!' said the actor-screenwriter. 'A boy who in 365 days has done more for American national health care than we could all wish to do in our lifetimes.'

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Family friend: Justin Theroux, 46, on Sunday took to Instagram to share a warm birthday message for his friend Jimmy Kimmel's son Billy on his first birthday 

Family friend: Justin Theroux, 46, on Sunday took to Instagram to share a warm birthday message for his friend Jimmy Kimmel's son Billy on his first birthday 

Theroux said that Billy 'doesn’t even know yet how courageous he is,' and that it's 'an honor to know him and his incredible and equally courageous parents.'

Justin, who announced his breakup with Jennifer Aniston, 49, February after more than two years of marriage, is a close friend of the ABC late night ace, who drew upon his own harrowing experiences with Billy to emphasize in the importance of universal health care coverage on his show. 

The Girl on the Train actor tagged Cedars-Sinai Hospital and Children’s Hospital Los Angeles at the end of the post, imploring his fans to '#registertovote' in an accompanying hashtag.

Kimmel on Saturday took to Twitter to thank the medical professionals who helped maintain the health of his 'tough little boy,' and implored his followers to register to vote as a happy birthday wish for his son.

Good times: The bearded actor had a laugh with his friend's adorable son

Good times: The bearded actor had a laugh with his friend's adorable son

Brave boy: Billy has already undergone two heart surgeries to deal with a congenital heart condition

Brave boy: Billy has already undergone two heart surgeries to deal with a congenital heart condition

Kind: Justin said that the tot 'doesn’t even know yet how courageous he is'

Kind: Justin said that the tot 'doesn’t even know yet how courageous he is'

Billy's mom Molly McNearney posted a shot of herself planting a kiss on her son, writing, 'One year ago I pushed this little fighter into the world.

'He’s taught me true strength and joy. We promise to keep fighting for kids like you, sweet Billy. Thank you God for this light.'

Other celebrities who chimed in with kind words on the boy's birthday included Alec Baldwin, Ellen DeGeneres, Julia Louis-Dreyfus and Tom Arnold. Ellen wrote that Billy is 'one tough kid,' Arnold wrote that Billy is 'already a hero,' and Baldwin wrote, 'God bless your family, Jimmy.' 

Pals: The actor-screenwriter and late night host are good friends, and have vacationed together in the past 

Pals: The actor-screenwriter and late night host are good friends, and have vacationed together in the past 

Grateful: The late night host posted this adorable shot of his tot, and implored his fans to register to vote 

Grateful: The late night host posted this adorable shot of his tot, and implored his fans to register to vote 

Inspiration: Billy's mom Molly McNearney posted a shot of herself planting a kiss on her son, writing that he's 'taught [her] true strength and joy'

Inspiration: Billy's mom Molly McNearney posted a shot of herself planting a kiss on her son, writing that he's 'taught [her] true strength and joy'

Flood of well-wishes: A number of Jimmy's famous friends chimed in to wish Billy a happy birthday 

Flood of well-wishes: A number of Jimmy's famous friends chimed in to wish Billy a happy birthday 

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Justin Theroux shares birthday message for Jimmy Kimmel's son Billy

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