I think it depends. If he is actively trying to stop or not drink, and has shared that goal with you, then a light, "I noticed you didn't drink on Thanksgiving, way to go" might be ok.

But if he hasn't told you he's trying to stop / stay stopped, then I wouldn't mention it as I think it would make me feel more insecure, like maybe I was being monitored.

That being said, I really appreciated the family support I got when I was trying to quit. What that looked like, was my brother congratulating me when I got through milestones without drinking, but I raised the milestone with him. He also let me vent when I was struggling with cravings. And he was totally non judgemental when I relapsed, he just encouraged me to get back on the wagon. And this was most important to me, he told me he was proud of me, when I hit milestones.