Hugh Grant filmed a video message in character as his Paddington 2 villain to cheer up a four-year-old boy seriously ill with cancer.
Henry Bard and his father Graham, from Caythorpe near Nottingham, had posted a clip on Twitter showing them performing a dance from the movie.
The footage reached the attention of the actor, who reprised his role as Phoenix Buchanan to send a reply.
Mr Bard said his son’s eyes “lit up” when it arrived.
Henry has spent most the past 10 months in and out of hospital after being diagnosed with high-risk neuroblastoma – an aggressive childhood cancer.
They performed the dance from Paddington 2 – one of Henry’s favourite films – as part of a series of “reasons to be cheerful” videos filmed during the coronavirus lockdown.
In the film, Grant plays a washed-up actor who becomes a criminal and ends up in jail.
Reviving the character, he told Henry: “Your dancing is quite remarkable. I congratulate you from the bottom of my heart.
“I wish I could say the same about your dad’s dancing but alas I can’t.”
Mr Bard said: “I’m not sure who was more excited – Henry or me. His eyes lit up and he was over the moon.”
The pair’s other videos have included Henry miming to George Ezra’s Shotgun and dressing up as a firefighter.
“The reasons behind the lockdown petrified us – we were so scared of Henry catching something as he’s so highly vulnerable,” Mr Bard said.
“But to be home together for the past six weeks has been phenomenal. It’s the longest he’s been at home since he started treatment.”