Step Families Support Group
Traditionally, a stepfamily is the family one acquires when a parent enters a new marriage, whether the parent was widowed or divorced. This community is focused on many of the relationships and issues that may arise within stepfamilies where you may find answers from some members and you may help other members with their questions.
When I went through my divorce I explained it to my kids like it was a situation with a puppy.
I asked if it was the puppy's fault if his owners fought. If he wasn't being loved. I asked if it was the dog's fault because it did what puppies do and peed on the floor, etc, etc.
Your step son is a bit old for that explanation, but it may still help. Because it's obviously NOT his fault that his mom left and can't take care of him.
I would say that you need to make statements to that effect, as well as statements that you love him, even when he doesn't love you.
In fact you love him...more...because that's when he needs it. And that it's a love of choice not birth...
I wish you and him the best,
Patti