I'd be very concerned for him. Possibly he can keep it under control. What's his story? How serious was his drinking problem before? Why did he get sober in the first place?
I used to sometimes get NA beers when I was out with people who were drinking. I think in some desire to still sort of fit in. After a few times doing that, I realized I didn't really enjoy them, and they were just as expensive (or more!) as regular beers, so I stopped doing it. I know plenty of people who can continue to drink NA beers and wines without relapsing, but I also know a few who did relapse. That may or may not have had anything to do with what they were drinking. I don't believe it's a slippery slope for everyone.
Snowy, I'd just keep an eye on him, but you also have to realize you are not in any way responsible for any choices your bother might make. He's got his justifications for doing it all lined up, so he's gonna do what he's gonna do.