Firstly I would like to say that his kids love him and he dotes on them.
He is in the situation though where he has two children who he sees every Weds and every other weekend from Fri after school until Sunday bedtime. He is a hands on dad and does all the mundane childcare stuff as well as the fun weekend stuff (he does dentist, doctors, opticians appointments if they fall on his Weds or Fri). He doesn't expect me to do anything for them (other than to be nice to them) and he does all meals, bedtimes, bath times, washing etc. He never misses a weekend (even when he had surgery the day before) and pays above the CSA amount, plus uniforms, shoes, school trips and any other unexpected expenses.
The thing he struggles with is discipline - he wants to cooperatively coparent with his ex but his ex is practicing gentle parenting. She doesn't discipline or give consequences for bad behaviour and it has resulted in an 11yo who thinks it is acceptable to punch their 6yo sibling and a 6yo who is frustrated at being smaller (and bullied) and is explosive.
I have (gently) persuaded my DP that totally ignoring the behaviour is not going to work - the children frequently injure each other. Unfortunately his ex has told the children that their dad has no right to raise his voice and to ignore him. He doesn't raise his voice (unless someone is about to get injured) but the children have been brought up to believe that no adult should question them. To give an example the 6yo is in a football club and will explode if the coach tried to tell him to do something differently (like sliding tackles are dangerous and he must not do them).
Is there anywhere that is a little bit gentler on dads than MN where he can get support? He is open to advice but I don't think anyone telling him he's a shit dad his ex does enough of that is going to help his mental health. He just needs techniques on dealing with a child who explodes and cannot listen and an ex who thinks her parenting way is the only way. I tried to explain about parallel parenting and how kids adapt to two sets of rules but I think I am too involved for it to have much of an effect.
He split from his ex 5 years ago, there was no OW/OM, we have been together for 2 years and I have a good relationship with the kids for some reason they listen to me. We do not live together.