My son was the same way, but he has this one thing about him called competition lol I'm ashamed to admit but I used the heck out of it. When his older sister would come home with good grades I tell him about how good she did lol I didn't compare them I just bragged on her a lot the next thing I knew he was bringing them home too. I say stuff like your sister is so good at math, and bam there would come A's oh and I use the heck out of guilt too. He warned me that one of his high school math classes was really hard and he might get a low grade so he brought home a low progress report and I didn't do it on purpose but I was so shocked and kind of teared up worrying because I had convinced him to take this class and was feeling bad about it. The report card came home and he had a A lol I know its terrible
My son is in college now and I sent him off to college with a lot of peace of mind that he would do well, and he has. I've told him about my wickedness too, lol. He was ok with it
His attitude is why would anyone want to bring home bad grades.