K.M.
This is going to sound hard but, since his mother is his mother, the best that you can do is to be kind and caring to the boy, and show as much support and understanding to both him and his mother as you possibly can muster.
He will ALWAYS love his mother whether or not she is a good mother. If he gets any vibe, however subtle, from anyone, that she is somehow lacking, he will just not know what to do with that, and it might really throw him for a loop. There is nothing to be gained by weakening that relationship. And, you are stuck with her too. So walk a mile in her moccasins and see if you can't find a few things about her that are great, and focus on those things only.
So, funnily enough, by helping him love and honor HER, you will model for him what a truly responsible and giving person IS, and you will earn his respect and love too over time without taking anything away from what they have.