Great advice. Our son took a similar journey and will be attending the Thornton School of Music at USC for Jazz Studies/Composition in the fall. His journey started with the piano early and kept adding instruments (drums, bass, trumpet, French horn…etc) until he found guitar. He still does most of his composing on piano - but the guitar is his main instrument.
In addition to all of the school activities, all-state…etc, support any interest they have in playing gigs outside of school. Our son started playing gigs in the area when he was around 15 and by the time he applied for schools, he had played more than 70 gigs - most of which were with local professional musicians. All of the performances helped bolster his resume and helped him improve as a musician. There is no better practice than doing - especially when you’re surrounded by musicians who are better than you.
Summer camps are so important - apply for as many as you can. The camp audition process is really helpful and you get great feedback if you don’t make it. It’s a lot like college applications but you get a couple of summers for do-overs… And the camps you do attend will expose your kid to future friends that share his passion and will push and inspire him to do more.
Can’t emphasize enough the value of taking music theory classes and any other opportunities outside of the school system. Our son took classical guitar lessons and Jazz improvisation lessons from professors at our local university’s music department for a couple of years and it really helped.
Above all else, I can’t emphasize enough the importance of making sure your kid is having fun regardless of the direction they end up going in life. What they are passionate about in 9th grade may change - but it your kid is passionate about pursuing music as a career, they will show you. And if that’s the path, then go for it! The world needs more musicians… 