He can feel how comfortable Astarion is which is very different from the resigned terror of Arthur making such an insane jump. That's why he doesn't yell or scream or complain; instead, there's something utterly delighted to feeling Astarion manipulate the Weave, use his magic and his talents. It's a little like listening to Arthur recite poetry or the few times he's heard him play the piano. He's silent, just soaking it in and enjoying it for what it is.