Put the “grand mother” in her place and tell her to back off if that's what you want. If her son was too much of a selfish turd to take care of his responsibilities, and she supported him in this, she has no place doing this to you or your son, particularly if your husband has taken the boy as his son. My father was a pretty bad father and husband, it is something I have always despised him for, my uncle, my mothers brother, was ten times the father to me than my own father ever was; and though my parents devoiced when I was 12 so I knew my father, I'm more than happy to acknowledge that my uncle was more of a father to me, and I appreciate it more than I can ever tell him.
One day your son might want to know his biological father but Its my guess he will hold your husband in much higher esteem than his biological father. The kid needs stability growing up, don’t let a pi#$y grand mother jeopardise that for him, you or your husband, tell her to back right the f^&* off. Sorry if this is a bit blunt or seems rude even, I don’t mean to be, especially to you, but its something that my experience growing up taught me to feel strongly about. I watched my mother struggle to just keep a roof over mine and my brother's heads, we didn't even have enough food sometimes, but I still can only find love and admiration for my mother for doing her best, and for my uncle for taking me in when it got too much for my mum. If the “grand mother” had no support for you when it counted and just wants to be the “grand mother” when it suits her that's pretty selfish and she doesn’t really give a sh&* about you, your boy and your husband. Tell her to bugger off.