Learn Spanish (Latin America): Cuando el chico ve sangre, se desmaya.

Because this sentence describes a habit or a general fact. When you use cuando (when) to talk about something that happens regularly, like "Whenever the boy sees blood, he faints", you use the normal present tense for both actions.

Why is there a se before desmaya?

The Spanish verb for "to faint" is desmayarse. It is a pronominal verb, meaning it always requires a reflexive pronoun like se, me, or te. Even though we don't say he "faints himself" in English, Spanish requires the se to show that this sudden physical change is happening to his own body.

Why doesn't sangre have the word la in front of it?

Just like in English, you don't need the definite article when talking about an unspecified amount of something uncountable. He sees "blood" in general (ve sangre), not a specific puddle of blood (ve la sangre).

Could I use si (if) instead of cuando (when) in this sentence?

Yes! Because this sentence describes a predictable, automatic reaction, saying Si el chico ve sangre... (If the boy sees blood...) would express almost the exact same idea as cuando.