Asking him to call when he's going to be home after midnight is, indeed, asking no more than common courtesy. And it's no burden on his recovery. It's not as if you are demanding that he check in every hour so you can hear how he sounds.
I'd suggest something along these lines: "I think it's great that you are spending so much time working with your sponsor, and I'm not complaining about that. I do worry, though, if it's after midnight and I haven't heard from you. I don't sleep well then, because I worry that you've been in an accident or something. If you could do me a favor and just give me a call or send me a text to let me know how late you'll be, I'd sleep much better."
If he has issues with THAT, then suggest he bring those up with his sponsor. It's utterly reasonable for one spouse to tell the other if they are going to be unusually late coming home.