Severus is an incredible actor, and the worst, most enviable type of liar; someone who can keep up with his own machinations and who can do it without his hands shaking. But even he wouldn't be able to just start picking at one of the headmaster's Gryffindor golden boys if he knew what was going to befall the old man. He has no idea. With any luck he'll avoid being informed, but hey, maybe if Lupin has any luck, he'll get to watch Professor Snape have a very quiet nervous breakdown when it's explained to him.

"I don't yet know how the progression of time works in this place," he forces out after an uncomfortable stretch of silence, "but I'm keeping an eye on it. I expect you and anyone else wandering around with your bloody face to choke down the appropriate medication as it's available."

Meaning: yes he'll make that potion, yes even at twenty-five he's had a look at it and given it a go already. This isn't altruism, it's Severus protecting his damn students.