Each of the sixteen boys will have prepared for the solo. Experience shows that the ten-second notice minimizes anxiety of the chosen boy, and I suspect it restrains the disappointment of the other fifteen -- some of whom won't have another chance because they're aging out of the trebles.
Whatever goes through the mind of the chosen boy, I'm happy for him. It was my great pleasure to sing in a first-rate youth choir at that age. We had the same rather demanding choirmaster as the adults, we learned the same repertoire as the adults (no easy stuff for us), we practiced just as much as the adults, and once a month we sang at the 11 a.m. service instead of the evening service. Some children and teens in this choir were really talented. I wasn't one of those, but I worked hard at it and sang my parts reasonably well. Much of what I know about music today, comes from those years.
My business takes me to England often. I'm not an Anglophile in the classic sense; I find things to appreciate in every business destination, whether it's Tokyo or Rio de Janeiro or somewhere in between. But I will say with conviction that the English choral tradition is the best in the world, a national treasure. And tomorrow if you're free at that time, give them a listen -- for the first four minutes, at least.