The "Gravity" hitmaker thinks the 22-year-old singer is "remarkable" and they regularly trade new music with one another because they know they can rely on each other not to be given "sugarcoated" feedback.
Speaking to Zane Lowe for Apple Music, John said, "He'll send me stuff he's working on, and I'll send him stuff I'm working on. And we don't really sugarcoat it for each other. Sometimes we go, 'Cool.' Sometimes we go, 'Now that's one.' He is so honest."
The 43-year-old singer finds it "really informative" for his own work to know what Shawn is working on and the musical direction that he's interested in.
"We send each other everything we make," he explained. "It's really cool. As I sit here, I know what he's going for on his next record - which I find actually really informative for me as a musician to understand. If I can understand where the key players are going, it helps me understand where I would sit, given the records I make."
"You know who he reminds me of?" he mused. "He reminds me of George Harrison, in the sense that his spirit is immovable, and it's his, and it's honest. And that's very George Harrison to me."
"I remember seeing George Harrison on The Dick Cavett Show - obviously not when it aired. And he's not pushed around by the excitement around him."
Shawn - who first met John in 2016 - previously admitted he's learned a lot from his "badass" pal.
"He's one of my really, really good friends," he said. "The most incredible thing I learned from (him) is that no matter how much experience you have, you never are done learning; there's always something to be learned."
"John will send me new music and ask me what I think. When that happened to me the first time, I was so impressed that he didn't see himself as a master yet. He's always trying to learn and wants to get better; he always wants to grow with the people and not get left behind."
"He's a badass. He's just so smart and intellectual. He has a very interesting view on the world — a view I've never heard from anyone else."