Pharrell Williams sang his hit Happy to a sick young fan onstage at the Roots Picnic festival in Philadelphia over the weekend (03-04Jun17).

Eight-year-old Reef Carneson, who suffers from acute lymphoblastic leukemia and has been in the hospital since he was one year old, had dreamed of meeting his idol Pharrell, so cancer advocate Milk Tyson made a video with Reef last week (04Jun17) asking for the singer to have a video chat with his fan over FaceTime.

However, Pharrell went much further and invited Reef to watch his set at The Roots Picnic festival in Pennsylvania on Saturday night (03Jun17). However, Reef is too sick to travel by air so the 44-year-old arranged for a private car to bring him and his family to the show from Cincinnati in Ohio. He also arranged for them to stay in a five-star hotel and got them tickets and backstage passes, according to TMZ.com.

Video footage obtained by the website shows Reef and his sister going into a room backstage and meeting Pharrell, who says, "What's up? How you doing? Nice to meet you," and gives both children hugs and Reef's sister a handshake.

To top it off, Pharrell invited them out onstage and got the crowd to chant "Reef, we love you" before launching into his song Happy. They remained onstage for the song and Pharrell moved around the pair and even bent down to sing it to them.

Pharrell's festival appearance came the day before he flew to Manchester, England to take part in the One Love Manchester benefit concert, which was organized by Ariana Grande to raise funds for those affected by the bombing at her Manchester Arena gig, which killed 22 people.

He performed his Daft Punk collaboration Get Lucky and a duet of Happy with Miley Cyrus. Following the show, he wrote on Instagram, "MANCHESTER you are an inspiration!! Thank you @arianagrande for bringing the world together last night. #onelovemanchester."