The clip showed Brown appealing to Johnson -- who he considers part of his "aiga" (family) -- to help spread awareness about the harmful impact of domestic violence.
"Since you're part of the aiga, I thought I'd send you a gift," the cutie said, while he was shown wrapping up a She Is Not Your Rehab t-shirt to post to Johnson. The organization was founded by Brown's father, Matt Brown.
Brown explained that his dad and uncles wear the shirt to encourage men to "treat girls well". The idea was inspired by the little boy's nana growing up amid domestic violence.
"My dad says that it's his mission to help other men to heal now, so their wives don't have sad eyes like Nana did," Brown shared. "I think you are the strongest man on the planet that many men look up to. If you wear this t-shirt, then I think they will listen to you and we can help houses everywhere be violence free."
On Sunday, Johnson, 48, reposted the video, which he admitted had made him emotional.
"Just watched and this one just sat me down 😢🖤💪🏾," Johnson wrote. "A bit emotional watching this 5yr old little boy speak to me."
Johnson then penned a personal letter to Brown in the rest of his caption.
"Angelou, It’s Uncle Rock. Yes, I’m you’re 'uncle' because of course, all us Samoans are related 😂✊🏾," began Johnson, who is half-Samoan through his mother, Ata, and spent some of his youth living in New Zealand with her family. "I’m very proud of you, son. You don’t understand yet, because you’re too young but just know how courageous and inspiring you are to myself and our AIGA [family], but now - to the world."
"I admire your father, greatly for standing up and creating @sheisnotyourrehab," Johnson continued. "And to make sure his mom - your Nana - will never be forgotten. Encouraging men all over the world to treat our women with respect, love and most importantly, violence free."
Johnson warmly recounted how his own relationship with his grandma reminded him of Brown's bond with his nana, who is ill with lung cancer and was shown in Brown's video.
"The way you sit in your Nana’s arms as she speaks to you is the exact same way I would sit in my grandma’s arms when I was your age, while [she] said her prayers in Samoan to God, and then she’d talk to me afterwards just like she talks to you," Johnson wrote. "She would tell me the exact same things your Nana is telling you. Listen to her. And always remember her words."
"You stay strong, Angelou and keep listening to your Nana and your parents," he continued. "One day you will become the leader of your aiga and also a leader the world will admire."
Johnson signed off "All our alofa," (meaning, "All our love") from "Uncle Rock and our aiga 🖤." But not before he hilariously pleaded for Brown to tell his sister to think twice about the images of Johnson that she shows him.
"Tell your sister to stop showing you pictures of me wearing a fanny pack!!!! 😂😂😂🤙🏾," he wrote, before noting that he had asked his team to connect him with Brown's family.
After seeing Johnson's post, the young boy's dad, Matt, told The Hits' Jono & Ben that his mom had become emotional reading the Hollywood star's letter.
He also told the New Zealand Heraldthat there was "no one better" than Johnson to help spread the organization's message.
"I feel it's time for Pacific men in our communities to stand up and say no more," he said. "There's no one better to lead that than The Rock himself. For many of us he was a superhero growing up so I thought if he would support this then Polynesian men would listen."