Hi notsleepingwell,
I also said no to this question. In my opinion, you have to take the high road, especially when dealing with kids. I'm in a similar situation to yours in a lot of ways. I have legal custody of my niece and her mother, my sister, has told her horrible things about the whole family over the years. While I'm perfectly willing to tell my niece MY version of events, contrary to her mother's, I will NOT retaliate by saying bad things about my sister. Sometimes I'm so angry at her that I want to give my niece an earful... but that's just not a good idea for developing the long term positive relationship I need to have with my niece. She is growing up fast and quickly starting to see the truth on her own. I can't push her to do that any faster than she is ready to... or else I risk damaging our relationship. I'd venture the same may be true for you and your grandson. Let it take time if it has to, I say, and don't sink to the level of someone you don't respect. All the best to you and your family.