9th grade is still a bit early in the process to be worried about admissions to school, while it is always good to look towards the future,. 9th grade is still very early IMO to be worrying about that. There is such a thing as doing too much, and I think your S may be entering that territory, if he is bogged down in 9th grade with that much homework, it likely will only get worse as time goes on. With music there are always tradeoffs, a lot of instrumental kids homeschool because they find the time in school, the homework, the rigid schedules, and often the pressure from the school music teacher to do a wide variety of things (when school music directors see talented musical kids, they tend to glom onto them, and want them to do all these ensembles, pit band/orchestra, you name it, and it can get ugly if the kid balks), and homeschooling gives them the flexibility they need (it isn’t for everyone, there are a lot of reasons not to do it). My thoughts:
1)I would drop the school program, if the kid still wants to pursue the cello. While school programs generally don’t take the kind of time other things in music do, it is still slots in the kids day, it is still pressure. If he is playing in a pre college program, he likely is getting a lot of musical exposure there, much more than he would from a school program (to be blunt, a conductor at a pre college program at a conservatory is likely to be teaching a lot more music than a public school music director, or even a private school one). If his passion is composition, learning about ensembles, learning about the music, can be a big help I would gather (not claiming expertise in composition, the parents who have composition kids like spiritmanager and hunt are the knowledgeable ones.
2)I agree with others, if the kid’s focus is on composition, if cello and piano are auxiliary to that, I wouldn’t be so concerned about competitions and awards on them, and quite honestly,it sounds like your S is musically talented and the teachers are looking at him as a serious student on the instrument, and that is going to cause problems. They will expect more practicing, because they see your son as a cellist or a pianist, when he sees himself as a composer. It is true that a lot of composers were also excellent instrumentalists, but they also were in a different world. I think the ensemble experience that cello gives, for example, is helpful, but if the cello teacher demands too much, then it may be worth dropping it. If the piano teacher is pushing the kid to do competitions, or practice a lot more, and is giving the kid a hard time, then you may want to talk to the teacher and explain where the kid is with it. It is a real positive IMO that the teachers see your son like that, it means he is very musically inclined if they feel that way about him, but he has the right to determine his own future.
3)I think as time goes on you should also be aware that with music students, whether in a true conservatory (stand alone school, like Juilliard, NEC, Peabody) or music schools within universities or conservatories linked to universities (Bard which requires a dual degree, Music school inside a university or LAC (Michigan, Norhwestern, Rice, etc), have a different path than a straight academic admit. For example, in stand alone conservatories the whole EC thing, the whole having X AP classes, having the hyper gpa, the top SAT scores, will not help your admissions to the school, the admissions are based on the audition.
For a student applying to a music school inside a university, grades and SAT scores will be a lot more important than at a stand alone conservatory, they can get you academic merit aid, and also those schools require the student to be academically admitted as well as the audition. They do judge music students by a different standard, because they understand how hard the process of getting ready to audition is, so a music student heading into performance will be admitted with stats that might not get them admitted if they were applying academically only.
With composition, if the student is applying for a BA program in music to study composition, then the academic achievement might be important, a lot more than if applying to a BM in composition (and on that, I say may because I am not an expert on composition programs, the other parents can give you more information.
As has been discussed on this board a lot with music there are tradeoffs, but if it helps your decision making any he may be able to drop the level of academics in high school and be able to carve out more time from the academics and allow him more time for the music. The common theme is that if academics conflict with music that it may be better to lighten up on the academics, since if you get in academically but not into the music program it doesn’t help you much, and likewise, great SAT scores and 4.0 GPA won’t get you admitted into a music program (with composition, it is more about the composition portfolio if I understand correctly, but still, the student will need time to develop that portfolio in high school).
If, in a year, your son says that ALL he wants to do is music and he wants 30 hours a week of it, then certainly homeschooling or an easier school might be in order. Right now it sounds like adding a sport and lowering both music and stress would be a good idea.