The literal translation of "He's a student" is "Il est étudiant". After, the utilisation of a form or an other depends on the context of the sentence. For example, you could say something like: "Ce garçon s'appelle Paul. C'est un étudiant de 20 ans qui travaille pour payer ses études".

In the first case, you tell his job. "Il est étudiant" = "He is a student" In the second case you describe the person with a demonstrative pronoun "C' ". "C'est un étudiant." = "This is a student" "C'est un étudiant sérieux"OK /!\ WRONG "Il EST (UN) étudiant sérieux" WRONG In some case you can use both : "Il est étudiant mais il travaille aussi le soir." "C'est un étudiant mais il travaille aussi le soir." The sentence has the same meaning but the first one tell his job the second one describe the person.