Iron Man star Robert Downey Jr has recorded a new video with a terminally-ill Scottish boy, praising him as the "true hero".
The actor became a fan of Aaron Hunter after the eight-year-old asked him to support a charity pressing for more research into his condition, ROHHAD.
In the video, Downey joins Aaron and his family to explain the disease and urge people to donate money.
In an earlier film the star jumped in muddy puddles as part of a challenge.
I may play Iron Man, but Aaron is the true hero. Go to https://t.co/ywpvrsejuv to see the full story. Donate to @ROHHAD_Global and help Aaron and his friends fight on. Every $10 is a chance to join me at #AvengersInfinityWar premiere.#randomactfunding #findacure 🎥@kevinfordmedia pic.twitter.com/H7rqVIRX72— Robert Downey Jr (@RobertDowneyJr) March 30, 2018
Aaron suffers from ROHHAD (Rapid-Onset Obesity with Hypothalamic Dysfunction, Hypoventilation and Autonomic Dysregulation), a rare condition which causes children to gain weight and develop breathing difficulties.
There are only about 100 confirmed cases worldwide and currently there is no cure.
In the new video, posted on Downey's Twitter feed, he says: "Aaron Hunter is an amazing young Scottish fellow
"We were introduced and as I got to know him more, I just got caught up in the spirit of his desire to help others who have this extremely debilitating disease called ROHHAD."
Aaron's parents, from Alexandria in West Dunbartonshire, also appear in the video, describing how the Avengers star's support has made a real difference to their campaign for more research into the disease.
Aaron's sister Lauren adds: "Robert helping us is a really good thing because it's not just helping Aaron, it's helping all the families, and all the children with ROHHAD."
The “Muddy Puddle” challenge is live! Film yourself jumping in the biggest, muddiest puddle possible (fun & safe) for 33 secs. Challenge 3 people to do the same. Post and tag #GOMADJUMPFORROHHAD. No puddle no problem! Same rules minus the puddle. Donate at https://t.co/ywpvrsvUm3 pic.twitter.com/dioXyBVEpw— Robert Downey Jr (@RobertDowneyJr) March 28, 2018
Downey posted the video days after another social media film showing him jumping in muddy puddles as part of a ROHHAD awareness-raising challenge went viral.
Dressed all in white and wearing a red hair wig and tartan hat, often dubbed a See You Jimmy wig, he splashes around to Jump by Van Halen.
He then urges his Avengers co-stars - including Samuel L Jackson, Scarlett Johansson and Chris Pratt - to get involved by joining the fight against the disease.