K.H.
I was married to an alocholic. When I first left him, my son, who was about 6-7 at the time, was glad that we didn't have to live with daddy (he was not abusive, but my son saw him passed out on the floor, sofa, things like that). It was later that he begand to be angry that daddy didn't live with us. On one hand he understood, but he missed his daddy. Sometimes a child doesn't fully understand until he's much older. This same son is now 19, he was 16 before he could really accept and "wanted" to understand.
Patience, lots of love, and letting him know his feelings are ok and support...that's the best thing you can do.