To add to what Sessh is saying, if you target the body and cleave onto the leg, you will not receive bonus damage on the leg. It would be worth attacking the leg for doubled poison damage even if this weren't the case, but it exacerbates the issue. That doesn't seem to be what's pulling him down - instead, he seems to have abysmal uptime on the legs at all (#flurry, #RevealingStrikeontheleg).
SnD is really the bread and butter of our damage though, and should never ever have a low uptime unless we're away from all targets for an extended period of time. It should never be lower on priority than applying rupture, and if your rogue is having issues with flurry/SnD usage, he can just take rupture off his bars to make things less complicated without losing much damage at all.