I Love This Boy But How I Know He Loves Me Back?

Tell him then he'll let you know you will no from his reactions!

What you should do is, act casual and flirty around him, but not too flirty. Ask him some things about his life, and what he enjoys doing, let him know that your interested, but don't be creepy, and do not verge on being a stalker. If he likes you he will either be in your face all of the time and try to get your attention in an obvious way, or he will be shy, nervous and maybe even ignore you or be mean to you.

You won't know by anything he says.

By the way he looks at you! If he always smiles at you for no reason, it means he likes you.
That happened to me once.
One day my parents put me in a camp for kids to go at summer, (I am 9 years old) and we discovered that a boy I like was going there too.
And when I got there he was obviously there and guess what? ALL day the boy i like was smiling at me.I would be walking, looking at him and then he looked at me smiling.He would do that all day.So then I discovered he likes me.