What he isnt realizing is that checking out changes the focus to it being about him and not his poor mom who is the one with the illness who probably needing support. I understand triggers and if he is scared but that is what AA is for, to help cope through lifes ups and downs. Do NOT feel guilty.
Recovery is about not checking out when stress or tragedy strikes. I've knew parents who've dealt with cancer in their small child -- so if anyone deserves to check it it is them but they never did. They bore the pain. And they got through it.