He did go through his Harrowing, I'm fairly sure. That he wasn't killed by the Templars doesn't have anything to do with being Harrowed or not. The codex on him indicates that he always had First Enchanter Irving to thank for not being punished more harshly than he was (solitary confinement notwithstanding). I think it's more to do with Ferelden being more moderate and the templars under Gregoir simply being less prone to summary executions.
Anyway, you answered your own question. That his phylactery was in Denerim is proof enough that he was Harrowed, because otherwise it wouldn't have been.
Given his age, if he had never undergone his Harrowing, the templars would surely have forced Tranquility on him by now. It's literally the only get-out-of-your-Harrowing-free card that exists for Circle mages.