Achilles didn't get as much time with his mother now as he would have liked, but every moment was always sacred. She was a figure that he would always look up to in guidance, especially now when he had this child coming into the world that he really had no idea how to welcome. He'd expressed his concerns to her hoping that she could put him at ease, in between this tension with her and his father. He too had not been so generous in his welcoming of Peleus. Part of it was Achilles being terrified that he might slip and make the same mistakes and he did not wish that on Polyxena.
The muscles in his back tensed as he straightened up at Thetis' concern. "What is it mother?" Having never met Peleus beyond being a little boy cast off to Chiron with Patroclus, it was difficult for Achilles to have any outward knowledge of when he was near. His main concern was the fact that his mother was troubled and he was immediately on the defense.