Agree that he needs to continue to plug away at the math and science needed to complete a college prep curriculum looking forward to potential engineering school. Next year his ACTs and/or SATs will tell him (and you) if he’s college ready. There are engineering schools that take B students with a decent math and science ACT score.
In my experience you don’t motivate young male teens with threats or with trying to explain some cost/value equation, but your mileage may vary. My good friend just sent a boy off to engineering school this fall who had a C average in high school and a 35 ACT score. It happens.
If the problem is he’s not getting enough study time in, you might work it out where you “schedule” 1.5 - 2 hours of study time that has to be completed every single night - either right after dinner or right after sports practice or right after school. Sometimes that small discipline can help a slacker student, but you need to pick a time that works with the schedule so it becomes the weekly routine with little to no deviation and the “friends” get the message and get with that routine, too. Make the study time someplace in the home away from distractions other than music if that helps the student focus. That worked well with all 3 of mine who could get easily distracted and slack off in terms of completing homework, turning in homework, etc. That alone will turn a C to be B in most schools. We did it right after dinner because of sports so by 8:30 at the latest they were “free” to do whatever they wanted until bedtime. It also helped develop the “study habit” that they carried over into college.