Friends and family of Billy Green, who is being treated for Acute Myeloid Leukaemia at Leeds General Infirmary, urged the singer to send a message to "make his dreams come true".
Billy had just woken up after having a lumbar puncture and three lots of chemotherapy in his spine when he was first shown the video, but was over-the-moon as he started to recover.
Lewis is currently on his international tour, but promised Billy backstage tickets to a show once he was back in the UK.
The superstar joked: “I hear that you’re a big fan of my music, and it really means a lot Billy because I need the money!"
“I’m in Singapore at the minute. I’m sorry this is just a video message, but I’m on tour.
“We’re all rooting for you, we’re all behind you.
“When I’m back and going to play some shows, we should organise - if you feel up to it - for you to come to a show.
“It will be great to say hello. Keep going and I’ll see you soon!”
Billy's mum, Lisa, said: "He's a lot more excited than he reacts here, he's had a big smile on his face all day.
"Lewis is an absolute legend, he doesn't know how much this means to us all. I can't believe he's actually done it.
"Billy has already planned what he's going to wear to see Lewis, black jeans and a jumper with Lewis' face on it - or a tuxedo!"
Billy will have another lumbar puncture next week to determine whether his blood count has risen, and if it has come up he may be able to return home within a week.
The 11-year-old said he couldn't wait to show the video his friends, adding: "It made me feel so happy and excited, it's so cool.
"I can show off to everyone at school now!"