Evey,
In my opinion, when it comes to kids, all the other rules go out the window. It sounds to me like your stepson is looking for an adult he can count on. It's okay to be that person, to "show up" for him, to let him know that not everyone in the world will overlook what's important to him. I say, read him that story. Your AH gave up that special "thing" with his son when he chose the bottle over the child.
You can't save that precious boy from his Dad, but you can show him what unselfish tender loving care looks like.
Hugs,
posie