I suppose not, now that you mention it, they just happen to go together (giants, dragons, everything over L55 etc). I should have tried to Rapture him to verify it, but IIRC he just flat out resisted every mez.
He's definitely stun immune though. Kind of a retarded set of messages
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[Wed Jan 16 12:10:48 2013] Your target is immune to the stun portion of this effect.
[Wed Jan 16 12:10:48 2013] Dartain the Lost is stunned by scintillating colors.