The Emperor relishes in reminding Lelouch he is a prince because in doing so, he is reminding him of what he lost and at the same time reminding him of what he is, hinting at the closeness between them. Lelouch despises Britannia but can't escape his blood. As for the not referring to Lelouch as "his son" or a prince when he was addressing Suzaku, it was because he was addressing Suzaku. I find it very interesting that he chose to play to Suzaku's distancing himself from Lelouch by using "Zero" instead of anything else, not even Lelouch.
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