How fresh is your divorce? And were you the one to ask for it? Mind you, it took a couple of years for us to get to this point. It would not have worked directly after our divorce. So your ex might just need some more time to come around.
It really is best for the kids though, and your ex hopefully will ultimately see that. Especially if your kids start whining about staying at his place on his nights. They really do prefer to sleep in their own beds and have all their stuff. They might become the motivators to get him to see the light.
Hang in there!