There's a brief moment when Stiles lifts up into the motion, when his ass lifts up to press back against Scott as he feels him grinding into him. A moment when he lets out a breathy sound, eyes closed as he feels need play over him.

But he's an Alpha. He doesn't get pinned. He can smell the tang of his blood in the air and he lifts up, moving to throw Scott off him. If successful, it's likely Scott would land off the bed and Stiles would move quickly to cover him, grab and pin and touch him.