Yes, it is just like when we are first dating! Most people want what they cannot have. It is human nature.
Right now, he truly has the best of both worlds. He has an amazing wife who has given an open invitation to a delicious life. You've given him the slightest taste of what it could be like. Now, I think that by shutting off that invitation, and FORCING him to be on the outside looking in at what he is NOW missing, will force him to realize what he has lost in you. Let him sit on the other side of the window looking in, unable to join in on all the fun, and the great life you have created for yourself, your son and potentially a new man who will treat you great.
Have an amazign Xmas party, WITHOUT him. Let him see all the fruits of what you have sown, and he was not able to enjoy. Let him go back to that LAME O party that OW has thrown - complete without his precious baby boy.