I'm sure your mom knows something's up. My parents knew something was up, but then again, they knew my bf and could tell he was doing something that wasn't very good...
Anyway, your mom sensing something is up is good because she won't be as shocked when you tell her everything. It sounds like she'll be really supportive for you.
The news about your bf is great. I hope he keeps up with the program. How long will he be in the sober residency? I had a very hard time with the no-contact period and didn't really use it to my advantage. I just spent the whole time counting down until when I could finally talk to him again. I think you're going about it in a great way and with the best attitude.