Learn Russian: Свежий сок полезный, поэтому я покупаю его. - Fresh juice is healthy, so I am buying it.

Поэтому means "so" or "therefore" and introduces a result. Потому что means "because" and introduces a reason. To use потому что, you would have to flip the sentence: Я покупаю сок, потому что он полезный (I am buying the juice because it is healthy).

Doesn't его mean "him"? Why are we using it for juice?

Его is the direct object (accusative) form of the pronoun он (he/it). Because сок (juice) is a masculine noun, you must use masculine pronouns to refer back to it. Even though juice is an "it" in English, Russian grammar requires the masculine pronoun here.

Where is the word for "is" in "Fresh juice is healthy"?

In the present tense, Russian almost always leaves out the verb "to be" (am, is, are). The first half of the sentence literally says "Fresh juice healthy". You just put the subject and the adjective next to each other!

Why does свежий end in -ий but полезный ends in -ый?