Nothing works - and of course it doesn't, that's the point. But when he hears Byleth's urging, it's a reminder of why he asked for this in the first place - to prove that he can do it. To prove that he's stronger than his urges. He grits his teeth and clenches both fists, closes his eyes and tries to lose himself completely in simple sensation, push himself into climax by letting himself just feel. He spends so much time trying not to that to be driven into a state where he can't do anything else is a kind of release on its own.

It's not much longer before every exhalation is a moan and his whole body shudders. Finally, with a particularly hard thrust from Byleth that hits exactly the right spot, he lets out a breathless, wordless cry and comes, back arching, fists pulling against the ropes as hard as he can and shaking with the intensity of it all.

When it's over, he collapses back to the mattress, boneless and panting hard, barely cognizant.