A book on video game geometry is probably not going to help your brother much. It will talk about mathematics topics used in video games, not necessarily those used in his curriculum, and may entirely skip over certain things he needs to know to pass his subjects. I also consider it a bit misguided to assume that because he likes games, he will like a game-related maths book: it is probably not going to be any more fun or explain things better than another book on the same topic.
Have you tried pointing your brother to Khan Academy?
It contains almost the entire American school curriculum and has an entire section devoted to geometry. If your brother is struggling because he doesn't quite understand how to do things, rest assured the Khan Academy's lectures have short and easy to understand explanations of just about everything.