You can't make him do anything. Decide on the best course of action for you and the DC, and stick to it. If he tries to sway you, stand firm. My exH was very resistant at first and it took months to bring him round and get a reasonable amount of co-operation, but I got there. He told me I had ruined his life, but even our 14yo DD commented recently that we are both much happier now.
My ExH wanted us to see a marriage counsellor, but I refused as it was way past mending. He phoned one who told him "if she says it is over, then it is over". From that point he started to see sense. I don't know who she was, but if I ever met her, I would hug her!
3.5 years on, we communicate mostly by email, we sorted out contact and child support between us, although I had to go to court to get a reasonable financial settlement. We are not friends, I don't want to be. He makes comments about our relationship, in fact he recently used his GF's relationship with her husband (who lives next door to her) as a shining example of something we should aspire to and about it being a good role model to the DC
. My reply was "Your GF's relationship with her husband is of no interest, concern or relevance to me."
I am trying really hard to get my DC to learn what a good relationship looks like and to insist that when they are old enough to start dating etc, respect is crucial. They had a crap role model when I lived with their dad and living separately has definitely turned out to be the best thing for all of us. Even exH would have to admit he is much happier now although he wouldn't, because to do so would be to admit that I was right.