Do I ask him about it or should I just let him do whatever, since I can't stop him anyway.
You are not "letting" him do anything and yes, you can't stop him. You can only decide how you may react, or not, to this information.
Is this a boundry you've set and now have to uphold?
I used to ask questions like this, but I realized that I usually knew the answer. Somehow though, I wanted to force him to answer. I knew he would probably lie, which would re-enforce my anger, or he would shamefacely admit it (and carry on the same way), which would re-enforce my victimhood.
The question had more to do with me, than him or his drinking.
I know, it's tough, but I have to keep looking at me. Somedays, that kinda sucks. Alanon helps alot!