There are many ways to say "You're my everything" in Spanish to a boy. Here are a few examples:
* Eres mi todo. (You're my everything.)
* Eres todo para mí. (You're everything to me.)
* Mi vida eres tú. (You're my life.)
* Mi mundo eres tú. (You're my world.)
* Eres mi razón de existir. (You're my reason for being.)
* Te amo más que a nada en el mundo. (I love you more than anything in the world.)
* Eres el amor de mi vida. (You're the love of my life.)
* No puedo vivir sin ti. (I can't live without you.)
Which phrase you choose will depend on the level of intimacy you have with the boy and how you want to express your feelings. For example, if you are just starting out in a relationship, you might say "Eres mi todo" or "Eres todo para mí." If you are in a more serious relationship, you might say "Mi vida eres tú" or "Mi mundo eres tú."
If you want to make your statement even more special, you can add a personal touch. For example, you could say "Eres mi todo, mi amorcito" (You're my everything, my love) or "Eres mi mundo, mi príncipe azul" (You're my world, my Prince Charming).
No matter which phrase you choose, make sure to say it with sincerity and conviction. The boy will be able to tell that you mean it, and he will appreciate the sentiment.