Now that just wasn't fair. Sirius knew that he couldn't say no to that look, not easliy anyway. It was easier to hold his own with James and Peter on the watch as well, he could say no to the three of them quite easily, but now, with it being just the two of them, he was cracking. It was true though, what were the chances of them having this sort of opportunity again? He couldn't cave that easily though.
"I don't know," he shook his head just slightly, making a failed attempt at taking his gaze from Sirius. He was going to argue his usual case, that he could have fun without getting plastered with the rest of them, slipping easily into his lecture about over drinking at their age, but it didn't come as it usually did, so automatically, not with Sirius looking at him like that, his hand at his arm...he was cracking.
"Alright, alright," he sighed, unable to keep from smiling in spite of himself. "If it will keep you and James off my back all night," he added, though he could care less what James wanted him to do most of the time. He didn't want to ruin Sirius' fun, not tonight. What was the worst that could happen? With Sirius there he knew nothing could go terribly wrong.