or "bête" instead of "stupide"
Con instead of stupid should be accepted
con is vulgar. I'm not surprised it's not accepted.
"Il a fait peur à moi" is not an acceptable equivalent?
memory lapse: we use il est here instead of c'est because of the modifier vraiment? Somehow I thought it was the other way around?
No. vraiment is not involved. c'est means it is or this/that is. It's impersonal.
In the sentence, we are talking about a male person (he). Thus, it translates to il in French.
You only have to say "c'est" instead of "il est/elle est" before a possessive adjective.
C'est mon cousin－✅
Il est mon cousin－❎
So this rule doesn't play a part in this sentence here.