You just described my son—piano, cello, choir!
Just from my observations, the ones who pursue performance are going to begin to more singularly focus on one instrument at some point. Even my son, who decided on music Ed instead of performance, and absolutely refused to narrow his focus to one thing until his hand was forced in college, couldn’t continue to do everything at the highest level required for a performance major.
That said, I don’t disagree with anything @thumper1 or @compmom has said, especially to let your son do what he enjoys.
My son started on piano at age 5, and by the time he was in 8th grade(ish) had won some piano competitions, shown some ability at composition, and was playing the earlier sonatas. He had added bass in 5th grade, switched to cello in 6th and was in a youth symphony. He joined choir in 8th grade, and surprised us all when he made all state choir in 9th. It was around then that he dropped piano. He quit lessons (I was so upset but he had realized the time commitment necessary to continue to progress on those long, difficult pieces and decided to move on) and didn’t play at all for a year—when he began playing again he still played very well, and difficult stuff, but for pleasure. He continued on cello and voice and did very well. He was in high school orchestra and two choirs (incl. vocal jazz), and a youth orchestra. We spent a lot of effort trying to make all this and solid academics fit into his busy life at a regular public high school.
He auditioned into college on both cello and voice, because he would not choose between them. This made making a college list, and applying, and surviving audition season extra hard. He was accepted on both instruments though. I don’t recall all the details but he had performance acceptances for voice, and I can’t recall if the cello acceptances were performance or music Ed or both. He pursued choral music Ed as a major—he finally had to choose a primary and secondary instrument at this point and had a choral scholarship so chose that. In college he took cello lessons for two years as a secondary instrument and has had a lot of accompaniment opportunities, and continues to play. He managed orchestra for part of one semester but that was only possible because the pandemic had so rearranged things, temporarily. For piano, he tested out of all class piano and has also spent some time accompanying others and continues to play.
The benefits of his multiple instrument background are huge. He’s a sight reading pro, theory wasn’t hard, he already had a large body of knowledge when he hit those music history classes. But the thing that has been most noticeable is that he has played Bach piano inventions, the Bach cello suites, and sung Bach in the baroque choir. He really gets Bach. That broad knowledge of music in general and the ability to make connections between all his areas of study has really helped his musical growth.
So let your son explore it all now. All the knowledge and experience he gains will be valuable. If he’s serious about performance he will likely begin to focus on what he wants most to do and maybe he will have to adjust the time he spends on each to focus on his main thing. Or maybe he will find a different focus as he goes along. One of my kids was set on being an artist until 11th grade when she told us she didn’t want art as a job and would be an engineer instead. She’s now an excellent engineer who does some quality art in her free time.
I didn’t want high school to be turned into preparation for applications. Too stressful.