Believe me, the kids understand.. This is exactly how my father was when we were these ages. They do not care about the girlfriends. they know their father loves them, but they feel sorry for him because he is so lonely he cannot be alone. This behavior will actually not make the kids needy or promiscuous later on, instead they are going to realize, they need to be happy with themselves, before they can be happy with others.
Never underestimate children..
Here is what you can tell your ex.. The kids do not care about the girlfriends. What they care about is being able to spend time with their father.. girlfriends are ok, every once in a while, but the kids need lots of alone time with dad.
All you can do is remind him, or if your kids feel like they can tell him themselves that is even better.
We never felt like we could tell our dad. But our mom mentioned it to him.. He of course assumed she was trying to interfere, but when he asked us, we told him yes, we did want to spend time alone with him.. Or for him to come alone to our school events.
Hope this helps..