How old is your son?
Can you put the counter position into an age appropriate explanation.
For example, explain that some people might not like you being friends with X, but you enjoy X's company because they are into dinosaurs and no one else really is. It isn't right for someone to say you can't be friends with X when X is fun and has done nothing wrong.
Because on top of your ex demanding full control over your relationships, if you don't meet new dad again it might illustrate to your son that controlling others works and that isn't good.
Reassure your boy that you would never have a friend who was horrible to you or them, that life is more fun with other people in it, that daddy can live his life his way but that is not your way.
You need to counteract the poison your ex is spouting, might take time, gentleness, but needs to be done.
As for the ex you need to firmly say again and again and again if need be, we are no longer a couple, you have no right over whom I see, as I have no say over who you see. End of discussion ex. I don't care what you think anymore. And repeat I don't care what you think anymore.