Perhaps if your AH had kept HIS MOUTH SHUT, instead of guzzling his beer as he has, he would not have any worries about custody issues.
Your stepson going to Alanon is NOT the reason for AH perhaps losing custody, It is the drinking done by dad that is the cause and concern for you all.
Lashing out and blaming anyone and anything for what is their own resposibility, is the usual response by an A, who is desperate to keep their actions out of the firing line. It is sheer fear at losing their ability to keep on as they are, that fuels their manic words and behavior.
I hope your stepson continues with Alanon and reading in the forums, to give him the tools and strength to cope with his father's problem drinking.
Frankly, I admire you for supporting him and showing him the way.
God bless