He is well aware of any legal action he could take, but this could very well end up with what he doesn't want; his kids being without both parents in close proximity to them and on a daily basis.
It's truly a power and control situation, bordering on domestic abuse (minus the physical violence) in my opinion. But I do not even come close to blaming her fully for this. He has allowed much of this and so needs to own up to what he can/should try to do and remedy the situation.
It's a clusterf***.