The royal pair now have an incredible bond, and one moment in particular really sticks out.
Charles knows his eldest son is protecting the monarchy, and will one day follow him onto the throne.
But we saw for ourselves how close they were back in 2019.
During ITV documentary, Prince Charles: Inside the Duchy of Cornwall, we witnessed Charles open up like never before.
They filmed at the Royal estate which he has built up over the last 50 years into a billion pound business to support himself.
Viewers saw William talking to a second-generational farmer, Mervyn Keeling, about his private estate.
William was thinking to the future and said: “I’ve started to think about how I will inherit the Duchy one day and what I do with it.
“I think it’s really important, the family angle, I really do.”
Viewers were then shown a clip of Prince Charles, who reflected on the conversation between his son and the farmer.
The Duke revealed: “When I saw it, I couldn’t believe it.
“I was deeply touched and moved by what he said. Frankly, it reduced me to tears. It did, really.
“Because, I suddenly thought, well, just hearing that from him has made the last 50 years worthwhile.”
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William’s wife Kate Middleton has reportedly been instrumental in bringing the two closer together.
The Duchess of Cambridge has made sure Charles is present for their children’s birthdays and key moments.
Royal commentator Camilla Tominey told The Telegraph: “The warmth which so obviously zings between father and son seems to signify a laying to rest of the ghosts of William’s childhood and any soreness he has felt towards his father after witnessing the traumatic fallout of his parents’ separation as a 10-year-old boy.”