"We don't try to get too excited about things, but there's definitely a sense of relief that he's done it," the teen's father, Jim, told PEOPLE from the family's home in Snowbird, Utah.
Collinson managed to make it to the 16,067-foot-high summit of Mt. Vinson, the highest peak in Antarctica, several hours before speaking with his dad. Despite powerful gusts and subzero temperatures, Collinson unhooked himself from his climbing partner and skied off the top of Mt. Vinson to a campsite several thousand feet below.
Collinson, who was home-schooled by his mother and is a champion freestyle skier, began his quest for the Seven Summits a year ago when he climbed Argentina’s Aconcagua (22,841 feet). Five months later (in May 2009), he stood on top of Mt. Everest (29,028 feet).
"He used to dream about standing on top of Everest ever since he was three," says his dad, an avalanche safety expert. "He'd picture himself standing up there in his windbreaker and tennis shoes."
Regarded as one of mountaineering's most extreme challenges, only about 200 climbers can say they've bagged the Seven Summits. "This was difficult for him," says his father. "But he wanted it bad. It was the culmination of years of work and dreaming."