Describes my son who also has ADHD and is math-focused. He took AP Calc BC junior year along with AP Physics 1 and did well in both. He's now taking AP Physics C (much harder to do well--very challenging but is his favorite class. Because of his love of math, he's planning to major in Physics in college and these classes sealed that decision for him).
It's possible that instead of being lazy, he might be exhibiting signs of ADD. My son, who has ADD diagnosed at 6, has a friend who wasn't diagnosed until HS. He was getting straight As through middle school (ADD folks tend to be very bright and can get by on their smarts for a long time) but grades fell when the demands of HS classes (where, in the DC area, many classes resemble college level courses) became more pronounced. That's when the cerebral cortex (less developed in ADD brains) didn't kick in to help manage the higher-level challenges that HS brings. I'd have some compassion and seek help first to rule that out. It's common (and the description "lazy" is frequently used for ADD kids who simply have undeveloped frontal lobes).