Sam took the fact that Dean hadn't pulled away as an sign of encouragement. Hey, he'd warned the guy and if he wanted to ignore the warnings that was his choice!
His grip of Dean's hair tightened, tugging at the roots. His whole body was writhing and squirming. He couldn't help it. He was so close and so wound up like a coil ready to snap.
He tensed, holding Dean's head in place as he threw his own head back, crying out in pleasure as he came, spilling himself into Dean's mouth.