Stiles is quick to comply, shrugging out of his hoodie and then pulling his t-shirt over his head. It's not like he hasn't been naked or nearly naked with Derek before, but this situation seems charged in a different way, and he should probably not be getting hard from it.
He twists his t-shirt in his hands but otherwise holds still. After a moment he lifts his chin a little bit, subconsciously submitting by baring his throat.