Ruined From Behind 3 Times in a Row Face Down Ass up

My sub is on all fours face down ass up with his balls tied up behind him and a spreader bar between his ankles keeping his legs apart. With his cock and balls on display behind him I take advantage of the easy access and milk his cock between his legs, edging him close to orgasm and frustratingly ruining it at the last moment 3 times in a row, leaving his cock to twitch and fire out the spunk unattended below him and give it a few good slaps of encouragement. After the third ruin he is very sensitive and throbbing but I keep stroking the cock while he moans and winces for some post orgasm fun. To end the milking session I plug his ass and leave him restrained for his cock to calm down to a suitable size for locking back in his chastity cage.