How can we overcome a controlling, manipulative grandmother, without hurting the child involved?

I was in a serious relationship with a woman who has an 8 year old son. (His father had passed away.) Now, I’m a father of five from my first marriage, with my youngest already in college. I don’t think I was prepared to take on the responsibility of raising another child at first. She (rightly) felt I wasn’t involved enough with her son, and broke up with me two months ago. I completely understood, and really respected her decision as a mother.

I’ve since realized where I went wrong, and am now committed to helping her raise her son. The two of us still love one another and we want to try again. Enter my ex-girlfriend’s manipulative mother… who provides a lot of good to her grandson (love, after-school babysitting, monetary support, vacations, etc.), but always with strings attached. She says if we get back together, she will withdraw from the child’s life. My ex is understandably afraid that our getting back together will hurt her son. How do we handle this situation, without hurting the boy?