I think it has to do with how often you use the Illithid powers in dialogue.
That would explain why I havent seen him yet either because ive kept myself from using it all but once I think.
Yeah I spammed that little bugger every chance I got. There seems to be a narrative dissonance, with the companions urging us to find a way to remove the tadpoles ASAP and not use them, but we get rewarded with more story if we do use them and the story telling us the tadpoles aren't transforming us any time soon.