i imagine part of it is just good ole habit.......we do what we know. you've engaged with him for a long time, and even the crazy is familiar. i like the term "uncomfortable comfort zone" - it's itchy like a wool sweater on a 90 degree day, but dammit, it's OUR sweater!!!
you can start limiting the time you allow yourself to interact with him. if he calls, set a timer, 5 mins, 8, 2, whatever you can handle and when it goes ding, you get off the phone. and if he starts on before the timer goes off, you get off the phone. if it's in person, you limit that as well. pick a day of the week for that.
we have to teach ourselves new habits and new behaviors. and it takes time. hang in there.