atleast communicate to him that you are concerned about him and knows reponsbilities towards him.You have made all arrangment to make his life secure eventhough you have remarried. Make him understand the situation so you had to take a such decision. try to fill up the gap of father but do not try to take the mothers space away by telling him how bad she is. for him his mother is struggling to give him good life.
Connect with him and give him all the assurance that you are there for him alway.