Robert Downey Jr. made an ill 8-year-old fan’s dreams come true as he travelled to the U.K. to surprise the boy before launching a new charity challenge.

The 52-year-old “Avengers” star took to Instagram to kick off the “Muddy Puddle” challenge, which saw him post a clip of himself jumping in the muddiest puddle possible for 33 seconds, all while wearing an all-white ensemble.

The actor launched the challenge to raise awareness of young Aaron Hunter’s rare illness, ROHHAD (known medically as Rapid-onset Obesity with Hypothalamic dysfunction, Hypoventilation and Autonomic Dysregulation). There is no cure for the potentially fatal disease.

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The “Muddy Puddle” challenge has been issued… Film yourself jumping in the biggest, muddiest puddle possible for 33 seconds(keep it fun and safe) Then challenge 3 people to do the same… Please post the challenge to your social media platform and tag #GOMADJUMPFORROHHAD… No puddle no problem … Follow the same rules minus the puddle… Remember have an awesome (safe) time… To donate click the link in my BIO or go to crowdrise.com/rdj Calling out my fellow OG homies @gwynethpaltrow and @jonfavreau #OGAVENGERS #ChrisEvans #scarlettjohansson @chrishemsworth @markruffalo @doncheadle @renner4real @samuelljackson @paulbettany @elizabetholsenofficial #anthonymackie and to the rest of the #MCU @prattprattpratt @brielarson @wongrel @chadwickboseman @danaigurira @davebautista #beniciodeltoro #benedictcumberbatch #kevinfeige #bradleycooper #loudesposito @evangelinelillyofficial #martinfreeman @wellhayley @idriselba @joshbrolin @karengillanofficial @letitiawright @pom.klementieff @jamesgunn @therussobrothers @thejudgegunn @imsebastianstan @therealstanlee @marvelstudios @twhiddleston @tomholland2013 @vindiesel @winstoncduke @zoesaldana #paulrudd ( 🎥 @jimmy_rich 🙌🏼 @mo_freek ) special thanks to @vanhalen for your continued inspiration… and to @rohhad_association_charity for your love and kindness #randomactfunding

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Aaron’s condition affects approximately 100 children worldwide and means he has to have a ventilator to help him breathe, which he calls his “superhero mask.”

As well as sharing his own vid, Downey — who was first made aware of the Scottish youngster’s condition via Twitter — also challenged his fellow “Avengers” co-stars Chris Pratt, Samuel L. Jackson and Scarlett Johansson to help him out.

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The actor then kicked off a competition inviting fans to get the chance to join him at the “Avengers: Infinity War” Los Angeles premiere in a bid to raise extra money for the incredible cause.

See more details in the clip below:

Aaron made sure to thank his hero on social media, posting a series of messages including: