I would've said, looking up at him bluntly, "Feel free to have a drink. But if you do, I'm leaving. It's entirely your choice."
I'm the A in my relationship (1 yr sober) and I think that statement would be perfectly appropriate. It's taking responsibility for you, not him. It lets him make his own choice.
If he's serious about sobriety and recovery, he should know that just because he made six weeks doesn't mean he can drink again. That's stupid. (But it is a common way that A's justify a relapse.) Hopefully he's in AA and has some guidance there.