Co-Parenting Dilemma

Advice needed please...

"My partner and I split approx 6 months ago, we have a young child together. We have remained friends in order to raise our child amicably. Everything was running smoothly until my ex got a new partner. She is not happy about him coming over to see our son and is very against the idea of us talking, even if its about our child. 
I have been friendly when I have spoken to her and I allow our child to see his dad all the time, I have gone out of my way to be nice to her and for my sons sake, i'm putting any issues behind us and moving on.
I would like our child to have both parents involved in his life, but she is making it very difficult. It is having an impact on our child because he is not seeing his dad very often."

What would you do in this situation? Has anyone experienced anything similar that can give some advice on how to move forward?