Child Credit

So my ex husband and I share custody of our three kids however our youngest son is not my ex husbands biological child. He has a different father. When me and my ex husband split up for unforeseen circumstances all three of my kids stayed with their dad. We have joint custody of all three of them he just has placement bc they go to school where he lives.  There hasn't been a custody order in over five years now. So I was wondering if I tell him I want to claim my youngest can I do that. Like he claims the other two but the one who is biological his I was wondering if me and my current husband can claim him next year and can he stop me? Like if I say no doesn't that mean he legally can't claim him without my permission?