I know you probably don't want to hear this, but don't. You are WAY too young to be dating or even thinking about dating. There's no point in telling him because your crush will go away in a few months. Just enjoy being a kid while you still can. Your teenage years will be full of school-work and friendship drama. 

Only after the majority of that is over - say 16 or 17 - should you begin think about having a romantic relationship with anyone. You are not mature at 12 - you're only a child! Please, please, please, don't start dating this young and throw away your childhood. I know you want to feel more grown-up, but when you look back on this you'll realise how immature people still are at this age. It's better to think that to yourself and learn from your past self, than to act on it and remember how silly you must have looked.