“And I was about to decorate the ground with your guts. Apologies." Astarion muses as he realises that you weren't with the enemies, lowering his dagger.

Then, you tell him all you know - the tadpole will turn everyone affected into mind flayers.

“Turn us into - ha. Haha! Of course it will turn me into a monster. What else did I expect?” - he says.

You suggest that you should travel together in hopes of fixing the tadpole problem. He was ready to go alone, but going in a group sounds better.