"I think a lot of people don't don't see Charles as a doting grandfather, but he absolutely is," Nicholl told ET. "He's spent more time in recent years with the Cambridge grandchildren and he's very, very keen to have a close relationship with Meghan and Harry's children."
The private visit -- which came amid Queen Elizabeth II's Platinum Jubilee celebrations -- was the first time Charles had seen his 3-year-old grandson, Archie, is some time, and was the very first time he met his 1-year-old granddaughter, Lili.
According to Nicholl, Charles deeply treasures "these times with his grandchildren" and he is "quite sad that there aren't enough of them with Archie and Lilibet."
"I do know that Charles absolutely wants to start healing that fractured relationship with Harry," Nicholl said, adding that the private meeting was likely "very much the beginning" of his efforts to mend fences, and "paved the way for some more conciliatory talks."
"He loves his son but he wants to forgive his son for everything. He wants to repair that relationship and move on," Nicholl said. "A big part of that is him wanting to be a grandfather and see those grandchildren. It may mean a few more transatlantic trips, but he wants to enjoy this experience."
During Harry and Meghan's trip back to the U.K., Queen Elizabeth was also introduced to Lili, who is her namesake, in person for the first time. Harry and Meghan had previously spent time with the queen in April.
"Being with her, it was great. It was just so nice to see her," Harry said during an interview with Today’s Hoda Kotb that same month. "She’s on great form. She’s always got a great sense of humor with me. It was really nice to catch up with her."