I'm a sophomore and if your kid is anything like me, someone who has As and high Bs in every class except Trig and Honors Chemistry, just continue pushing him; don't let him give up. Sadly I think my mistake is that I didn't care enough about the two classes early on in the year. 2nd semester I started off with a 46% on a test, and that really put things in perspective for me. The way grades are weighted it was impossible for me to get an A in the class after that. I've scored 85%-95% on every test and all but one quiz since then, but still have a C.
I would let your kid know how important it is to try your hardest, even if he doesn't think it matters. Because the way your grades are calculated, you mess up one time and you can expect to be disappointed in yourself for the rest of the year.
As a parent I would give the same advice I wish I had the balls to tell my parents about: I don't care if i'm disciplined for bad grades, hell really deep down being disciplined is a good thing to me. But being praised for good grades is the one thing that keeps me motivated. I keep notes for myself in my locker or as reminders on my phone like "Keep it up, you can do this!" or "Make sure to study for chem. Working for B!" just to force myself to give a crap about the classes I don't like. I just wish once in a while my parents took me out to dinner, or even just gave some type of encouragement for good grades; because it's really what keeps me going. Yet we spend most of the time talking about my bad grades in a completely degrading way. The same type of encouragement can be used even when discussing bad grades. Instead of yelling at him for bad grades, how about you each get a separate talk time, no interrupting or being rude to stop fighting or yelling, where you can discuss what you each think the root of his problems are. I've asked my parents to do this with me and in the end we've found what I really need to work on, do in the future, and I feel actually encouraged by my parents to go do well in school.
Sorry if that seemed a bit rant-ish, but he might not want to tell you things like this so maybe I can help him out a bit.