BRAINTREE -- On July 19, Ari Davis of Somerville, soon to enter eighth grade at St. Paul's Choir School in Harvard Square, was treading the boards backstage at the Koussevitzky Music Shed. Andris Nelsons was conducting the Boston Symphony Orchestra and a star-studded cast in Giacomo Puccini's opera "Tosca" at Tanglewood.

Ari, 12, played the role of a shepherd boy whose song opens up Act 3. He's used to singing under the BSO baton: In fifth grade, he and his classmates, invariably billed as "The Boys of St. Paul's Choir School," sang in Stravinsky's "Persephone." Last season, they could be heard in Mahler's 8th Symphony and Korngold's "Die tote Stadt."

Playing the shepherd boy was different. This was Ari's first time alone on stage without his classmates. It is extremely rare for someone so young to sing solo with the BSO.

"I was very nervous at first," Ari said. "I was pacing, waiting for my time to go on stage, and I was very nervous."

Then, he said he saw Bryn Terfel, who sang the role of the villainous Scarpia, smile at him. Terfel gave Ari a "rush of confidence." He was still nervous, but he knew he'd be alright.

"I just smiled, and I sang, and it went by as a rush," he said.

Ari said that for singers like him, "it's all in the moment." Stepping on stage, all of the breathing techniques and strategies he learned disappear.

"It's almost like you lose consciousness," he said. "You go up there and you do it and it's done. The adrenaline is surging in you. You don't notice anything else."

He was the first one to walk on stage during curtain call.

"I was the first catalyst for the cheers," he said. "I was standing there, and the light was going in my eyes, and I was holding hands with the whole cast during bows, and it was an amazing experience. I can't even describe it."

St. Paul's, the only boys Catholic choir school in the U.S., didn't just teach Ari to sing. It taught him how to be confident enough to perform solo.

"Being at St. Paul's, you'll get more opportunities than some other people," he said. "I feel like everything culminated to that point throughout the entire year."

Ari's mother, Mariya Manzhos, often compares Ari's resume to what she was doing at his age.

"St. Paul's has had a huge impact on Ari as a person, as a human being, as a member of a group, as a member of a team, as a musician, as a student academically," she said.

Ari and his classmates sing Mass at St. Paul Parish every day.

"They get to spend time singing and serving, giving their voice in service to others, to God," Manzhos said.

She said that singing is a time to meditate, to "just be" without the distractions or demands of modern life.

"I find that singing offers musical training but also a very valuable part of his life that I hope will get him to know himself better," she said.

When Ari enrolled at St. Paul's, he had just finished first grade at a public school. The pandemic was raging, and his parents wanted him to be learning in-person. Manzhos liked St. Paul's small class sizes and responsiveness of the teachers.

"We went to their concerts, and we really loved the singing, and I wanted to learn how to sing," he said.

Ari is a vocal music fanatic. He loves "all the greats" like Mozart, Bach, and Handel. He likes to "dabble" in composing pop music on the piano, but classical music "has a whole other layer of depth."

"I love the expression and versatility of voice," he said. "I love the idea of being part of a choir because it's more unified than any other sort of thing. In my experience, I think it's a lot harder to be unified in something like an orchestra."

The BSO reached out to St. Paul's Music Director Brandon Straub, looking for a student to play the shepherd boy in "Tosca." The school's voice teachers rounded up a handful of candidates. Their auditions were recorded for Nelsons himself to listen to.

"I was excited to do it," Ari said. "I wanted to be part of the experience. It's such an amazing experience to be part of this large production with such amazing people. Even if you don't have the biggest part, it's still amazing to be there with them."

One of his friends auditioned, but his voice had already changed. He was relieved that he didn't get the part.

"Some were hoping for it, and others knew that in the audition, they realized their mistakes," Ari said. "They weren't quite as disappointed."

"Tosca" is the story of its title character, an opera singer whose lover, a painter named Cavaradossi, aids a dissident named Angelotti.

"She's a very jealous woman," Ari said.

The evil police chief Baron Scarpia uses Tosca to find Angelotti. Like many operas, the plot is a tangled web of romantic and political intrigue.

At Tanglewood, Ari's singing partners have been internationally known opera stars -- except for his understudy and classmate Charlie Kraman. Ari and Charlie hung out between rehearsals and got autographs from the cast.

"They were very kind to us," Ari said.

At his first rehearsal, Terfel gave him a thumbs-up and winked at him. There were no divas or prima donnas in the company.

"Everyone's equal in the production," Ari said.

After graduating from St. Paul's next year, he and his family will move to Utah. He wants to attend Brigham Young University or "any Ivy League schools" to further his goal of becoming a professional singer.

"It's not much pressure," he said. "There is some, but I don't really feel, it's not like a total struggle all the time. I love doing it, I love the music."