Hi Liz,
If I recall, your son has a learning disability. How much of his emotional life is impacted by it? You are still possibly quite responsible for his emotional maturation though he is 18.
He saw you put up with AH for years, then you finally broke away. Think about how hard you worked to get to the point you are. He may very well be unsure despite you talking with him a multitude of times what his role is with his father and what freedom he truly has a a young adult to walk away or put up boundaries. Seeing you work on your own financial and emotional health independently is very important visual messaging.
giving him the emotional tools to deal with his alcoholic father could be a very important tool kit that can aid him for a lifetime. Maybe you could participate in the counseling in a limited way to express your concerns about what seems to be going on during his visits with Alcoholic dad and also work through your plans to move in with a new family and the best timing of that move.
Is he going away for college? If so, maybe you could make that the time you move in with your other, 'new' family. That is a traditional launching time in our culture. And it shows for him that he is your primary concern until that time. Because if your son is mixed up, he could easily read into this adult relationship as you abandoning him just like you abandoned his father. Because tha it is probably 'the truth' of what your X will bitterly state. I think you need to consider working through all of it before you move with the counselor.
Take care,