It may help that Sam is never entirely silent. Each article of clothing is met with a sound or word of approval and praise. Sam's never been stingy with praise, not since he realized just how much Castiel needed it.

He reaches down, palming himself through his jeans but not making a move to his belt or zipper. "Never thought just listening to you get undressed could get me hard, but it's working." He lets his smile and arousal bleed through into his voice. "When you're ready, go put your clothes in the hamper, then go put on your collar. The blue one, please." One that he'd normally put on himself, but which he couldn't. But it was his and it would serve as a connection between them, a clear mark of ownership and grounding for Castiel.