B's happen. Unfortunately. I wouldn't worry about it. It's great to have A's as the standard, but honestly, because my dad has never accepted B's, I don't tell him anything about my school grades. Even in college, he doesn't ask and I don't tell (Mom is completely aware of my grades because she's calmer). I have a 3.8 GPA. He doesn't need to know the specifics. Self-motivated kids don't need too much parental intervention. Just encourage him to improve, be that raising it to an A or B+
FYI, my dad just walked in and asked why I wasn't doing my homework. I was (hence the textbook). "Turn the TV off; you can't do homework in front of the TV." I'm in college; if I want the TV on while doing homework (which I did all the time in high school), I think I can decide that for myself. The TV is still on.