I'm glad you had a good night Jenni;
One thing that often happens with family members is that they buy alcohol when they go out to get the take out, pizza, etc.
and buy the alcohol, consuming it before coming home.
I'm not saying he did that, but many many family members do.
I think it is great you have started talking with him about this, but be aware he may just take the drinking more "underground" by hiding it from you.
His intentions may be to not upset you, but the underlying problem would still be there if that is the case.
Take care and please keep posting.