The winger fired his first league goal of the season in some style with a stunning 20-yard curler after being picked out by a pass from Michael Doughty after 20 minutes.
And that proved to be enough to send out-of-form Posh reeling to a fourth successive League One loss.
Stevenage saw it out despite having midfielder Jimmy Smith sent off in the 64th minute for collecting a second yellow card from referee Mike Jones after cynically body-checking Hogan Ephraim. The visitors also had five other players booked.
Ephraim, on loan from QPR, failed to convert Posh's two best chances. He was denied by a fine Chris Day save late in the first half and saw another good opportunity blocked by defender Luke Jones early in the second period.
Posh skipper Tommy Rowe saw a dipping 25-yard volley hit the top of the crossbar, but the hosts were unable to prevent their slump from continuing as they went down with a whimper.