Welcome to Mothering! This sounds like a tough situation. What has the counselor suggested?
In addition as it looks like your post might have been missed, I'm bumping it up for attention so others here can see it and offer their thoughts and support.
mir44, can you get yourself to a lawyer asap? This sounds like abuse and parental alienation syndrome, especially since you and your son had a good bond before. Your x could be feeding his gf (and your son) lies about you too. Good counselling with a counsellor who understand abuse dynamics and how this impacts parenting/children is essential. A local DV shelter might have access to some of those resources for free. Document document document....and don't share anything with your x other than the bare minimum for your son. Try to keep all communication with him and her in writing. Record your exchanges secretly on a device if you can. A lawyer might recommend renting a house instead of an apartment just to level the playing field.