Johnson posted a short video Monday to Instagram in which he wished Hyrum Harris well and sang a bit of his signature song in "Moana," "You're Welcome."
"This request came across my desk and punched me in the gut a bit," Johnson wrote in a caption for the Instagram post.
Quoting from the request, he continued, "There's a little 3 year old boy out there named, Hyrum Harris. Diagnosed with acute lymphoblastic leukemia which is complicated by Down Syndrome. In the fight of his life right now and doing his best to stay strong - by watching MOANA up to 10xs a day because the character he loves, MAUI makes him feel strong."
In the nearly 3-minute video, Johnson portrayed himself as another fan of the 2016 film and not the voice of one of its most memorable characters.
"I wanted to tell you, man, stay strong and there's another dude out there… me, Dwayne, who like you loves Maui, too," Johnson said. "There's no one like Maui."
Hyrum's parents, April and Rich Harris, told People called Johnson's response to learning about Hyrum "beautiful."
"This has been such a moving experience for our family," April Harris said. "Every time Dwayne mentioned Hyrum's name, Hyrum pointed to his chest and in his soft sweet voice repeated, 'Hyrum,' as though he knew this was special for him. … When he began to sing Hyrum's eyes got very wide."
The family has launched a GoFundMe campaign to help pay for Hyrum's medical care.