If dad is willing what you could do is see about having it so the kids alternate extra days of alone time with either of you, That way they get the extra attention they were used to the parental bonds are preserved and everyones nerves are assuaged.
It sounds silly but it makes for more stable children.
There are specific programs around the country for kids whose parents are going through a divorce (e.g., Banana Splits). Perhaps this is something that your son's therapist or the school could offer advice on. How much can you and his father work together to address the issues with your son? How involved is he in dealing with your son's problems?