Unfortunately, it sounds like your crush has developed a crush of his own on the girl in his class. When he was new to your school you may have hit it off, but now that he's getting more involved in classes, he is interacting more frequently with students his own age and therefore has more in common with them. Don't take this personally. Attraction is often a matter of frequency of interaction and similarity.
Besides, as a teen, three years is a huge age difference -- too much, I think. It's better if you stick to someone closer to your own grade level. I'm sorry.