Wait a minute. . .
It sounds like you like your boyfriend a lot, and it also sounds like he has a great family who cares about him very much and has a solid set of values-- focusing on grades and getting a good education is really important for his future, and any future you may have with him.
Obiously, they've done a great job so far raising him into the boy you like so much now, so why are you second-guessing their priorities after what-- 16, 17 years?
Encourage him to do well in Geometry, help any way you can, study in the library with him-- it will only do good things for both of you!
And definitely, resist comparing him to his sister-- do you want to date him, or her?