If he'd be an SCP article, I'd propose to label him as SCP-null just for thematic purposes, even if the numbers aren't supposed to be poetic.
He's IMO also fairly far from a Keter class, pretty much as far as possible and in either direction, to boot. On the other hand, if we were to go with the potential of escape and the worst-case ramifications of it, he'd be a shoo-in Apollyon (and ironically one of the rare not completely cringe ones), as there is no containing him and just ending it all seems to be something well withing his purview. On the other hand, he has been securely contained for centuries(?) without apparent major incidents and so far the concrete steps to keep him under watch seem easy, reproducible and reliable. That'd make him Safe for most considerations.