Hola Camille

You could use estéis too. In this case we used seas (ser) because the speaker is referring to an inherent quality of that person, as in he is genuinely a jealous guy, part of his personality. If you use estar, then you are referring to this person "getting" jealous in a precise moment for a specific reason, not necessarily meaning that this is the sort of person he is. It is a temporary thing.

I hope it clarified it.

Saludos

Inma