There is a time to teach and learn discipline, like learning multiplication tables and doing chores, and times where being a part of a community, being of service, or honoring a commitment is important.
I can't see forcing a teen to continue to play an instrument and remain in the band as one of those times. Didn't school just start a few WEEKS ago?

So he can't change his mind about courses he wants to take, or being apart of, that will affect him for the next TEN MONTHS?
He's 13, not 35. He doesn't have the sense of time the way adults do. What a lesson to teach him, about being stuck and paralysed with a decision and your time thereafter if you don't/can't think ahead into a future you can't fully see.

How about teaching him instead to honor his insides and find what he's really passionate about? Kids are expected to already know this at 13?
The clues were already there from last year since "it has
always been like pulling teeth to get him to practice." So exactly WHO is the one really wanting him to play this instrument and be in the band?
An instrument should be a JOY to play, not a punishment or obligation because someone didn't plan ahead correctly, or someone else is exerting pressure to get the person to play. It may also cause him to loathe to play the instrument he once liked, and never play it again once the "obligation" is over.
In elementary school, the band teacher wanted me to play the clarinet, as we had one, from when my brother played. I hated the idea. I never liked the instrument. I wanted to play the flute or the piano. I was
forced to take up clarinet.
Never practiced, and in two classes, when the teacher finally dropped me from class, I was soooo pleased!

But, I didn't get to learn another instrument, as both my parents & the band teacher said, I probably wouldn't be disciplined enough to stay with that instrument either.
SIX YEARS later, when I finally was allowed to learn and play piano, I played passionately every day for hours. There was no pushing on my parents part. I LOVED playing the piano.

Had I been allowed to play it six years before, I might have become quite an accomplished pianist. I lost many years due to other people's ideas of what I would and wouldn't do, and how disciplined I might be. I never considered the piano a discipline - it was a passion.
Recently, I relearned this lesson, again. I started ballet again, two weeks ago, for the first time in 20 years! I had been thinking about it on & off for a while, but I was locked into a gym membership that I barely used, that was making auto-deductions from my credit card for the last 3 years. The
only reason I joined a gym at all was because I was under the cultural hypnosis that it's the right thing to do, force myself to to to the gym for workouts.

I couldn't foresee how it would be for the next 3 years.
I finally realised, it was work to get me to go to the gym, whereas I love to dance! (I also want to try pilates & yoga.) But, I didn't have the funds to take any other kinds of classes during the auto-deductions. I can get the same resistance body conditioning that from the barre exercises that I would get from any gym machine. I used to happily go to dance classes 3 times a week without needing to
drag myself there, like I did for the gym. I looked forward to class.
NOW that my gym obligation is up and I'm passionately dancing again, I sooo look forward to classes. I even got a dance tape, so I can practice a few times a week since I can only afford one class a week.
I realized I was literally out of my mind; to think I used to drag myself to the gym to powerwalk on a treadmill with my iPod, because I had made an unforeseeable obligation and commitment to a gym membership, when I could have been dancing instead! I won't make that mistake again! Life is just too short.