Your son could learn musical instrument thr' school or not. However, must perform the instrument in school exam but choir does not count as an instrument.
You could replace the word "音樂" in your concern statement with any other subject, you will get the answer.
At the time when you choose YW, you already believed it will give you back a good child. Trust the school, you should also trust the whole system. Every piece of activities serve it's purpose in the overall child's development chain. Music is just one of the elements.