Ex asked if can claim 1 child as an eligible dependent. I am married. He is single. They go weekends. Is this ok if I have full custody? Can I claim the child for CTB?

Two children with ex. i've been claiming them both since born. I did claim oldest as equivalent to spouse a few years ago before re-married. Now ex wants to claim. I have full custody, but they go there 3/4 weekends a month. If ex is claiming eligible dependent amount, can i still claim the same child for child tax benefits and all other deductions?