Been there, done that, have the tee shirt
I came to this forum becuase I was in a situation very similar to yours - I started dating a man named Drew who was divorced with a son and a crazy ex-wife. Drew was a wonderful man and his son was great. My situation was different, in so far as I want to get married and have a family some day so I welcomed the thought of having a son.
Ultimately what it came down to as time wore on was that Drew's crazy ex was ALWAYS going to be in our lives. She was always going to wrecking plans by having "emergencies" that required her to drop off their son at Drew's house at the last minute. She was always going to be walking into his house, eavesdropping on phone calls, etc, etc. There was only one viable way out - one day I would have to out-bitch her, and I knew I COULD do it, but I didn't think I would like who I would have to be in order to do it. Things just sort of fizzled out . . .now I see a man who is more on the same page that I am and things are much better for me. Hopefully Drew can find someone who can handle the challenges that his ex-wife brings into his relationships.
Jules
I think the mistake a lot of us make is thinking the state-appointed shrink is our friend. - Jack Handy