@PragmaticMom, it’s hard on parents when two siblings are so different. (I have that situation with my two kids.) And I know that it’s hard to watch a child make poor choices – but hang in there. Our education system is very forgiving. If your son decides to rededicate himself to school in the future, he can still graduate from a good university. (And If he doesn’t want to put in the effort, maybe college is just not for him. There are many other ways to make a good living.)
I think it’s a good idea to cut off your son’s music lessons if he’s not practicing, but if I were you, I’d encourage your son to explore new hobbies. You never know what might spark his interest, enrich his life and, yes, make him reconsider school. One of my son’s friends was a failing high schooler until he got into working with his hands - metal and wood shop, etc. He ended up going to college for industrial design and has a thriving, successful business.