Hollywood star Robert Downey Jr flew to Scotland to help an schoolboy with a rare condition that causes children to gain weight and suffer breathing difficulties.

The Iron Man actor became See You, Jimmy as part of a viral campaign to raise awareness for ROHHAD, a disease so rare it only affects 75 people worldwide.

And eight-year-old Aaron Hunter, of Alexandria, Dunbartonshire contacted the star on Twitter and was overjoyed when Downey Jr invited him and his family to London for a meet and greet to raise awareness for the condition.

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Hollywood star Robert Downey Jr flew to Scotland to help an schoolboy with a rare condition that causes children to gain weight and suffer breathing difficulties

Known medically as 'Rapid-onset Obesity with Hypothalamic dysfunction, Hypoventilation and Autonomic Dysregulation', the condition is potentially fatal.

So hoping to help the sweet schoolboy and give him a day to remember, Downey Jr posted a video of his new Muddy Puddle craze to raise money for ROHHAD research.

Downey Jr – all in white and wearing a See You, Jimmy hat – danced in mud as Aaron and his family laughed. The Instagram video has had 5.6million views.

Aaron's mother Elisabeth, 37, said: 'This was by far the most magical moment of Aaron's life. It was awesome.'

The Iron Man actor became See You, Jimmy as part of a viral campaign to raise awareness for ROHHAD, a disease so rare it only affects 75 people worldwide

She told the Daily Record: 'Aaron is quite poorly at the moment and this has come at the perfect time.

'It has kept the smile on his face and given our campaign a huge boost.

'He is very happy and he is wanting to thank everyone for helping him to reach Robert and making his wish come true.'

And eight-year-old Aaron Hunter (right), of Alexandria, Dunbartonshire contacted the star on Twitter and was overjoyed when Downey Jr invited him and his family to London

Downey Jr appeared in a video uploaded to his Facebook page last week, confirming his support and explaining the competition, run in conjunction with his foundation.

He and his wife Susan founded Random Act Funding to distribute financial support around the world.

owney Jr appeared in a video uploaded to his Facebook page last week, confirming his support and explaining the competition, run in conjunction with his foundation

Through it the actor is offering fans the chance to attend the world premiere of 'Avengers: Infinity War' with him in California on April 23.

He will fly the winner out and put them up in a four-star hotel for two nights.

Fans must donate $10 for a chance to win. Visit http://www.crowdrise.com/rdj to enter.