Apparently Astarion has a special dialogue option if Tav is already a Slayer the day after they are turned. I was so pleasantly surprised to have an option to tell him about Tavs special form.

I wonder if it is tied to the fact that he never saw the new form before turning Tav, or if it is there as long as you have the Slayer form? I'm guessing the latter.

I love details like that in the game!